Causes of mental disorders

From Wikipedia, the free encyclopedia

 

A mental disorder is "a clinically significant behavioral or psychological syndrome or psychological pattern that occurs in an individual and that is associated with present disability or with a significantly increased risk of suffering, death, pain, disability, or an important loss of freedom."

The causes of mental disorders are regarded as complex and varying depending on the particular disorder and the individual. Although the causes of most mental disorders are not fully understood, researchers have identified a variety of biological, psychological, and environmental factors that can contribute to the development or progression of mental disorders. Most mental disorders are a result of a combination of several different factors rather than just a single factor.

Research results

Risk factors for mental illness include, psychological trauma, adverse childhood environments, and genetic predisposition and personality traits. Correlations of mental disorders with drug use include almost all psychoactive substances, e.g., cannabis, alcohol and caffeine.

Particular mental illnesses have particular risk factors, for instance including unequal parental treatment, adverse life events and drug use in depression, migration and discrimination, childhood trauma, bereavement or separation in families, and cannabis use in schizophrenia and psychosis, and parenting factors, child abuse, family history (e.g. of anxiety), and temperament and attitudes (e.g. pessimism) in anxiety. Many psychiatric disorders include problems with impulse and other emotional control. 

In February 2013 a study found common genetic links between five major psychiatric disorders: autism, ADHD, bipolar disorder, major depressive disorder, and schizophrenia. Abnormal functioning of neurotransmitter systems has been implicated in several mental disorders, including serotonin, norepinephrine, dopamine and glutamate systems. Differences have also been found in the size or activity of certain brain regions in some cases. Psychological mechanisms have also been implicated, such as cognitive (e.g. reasoning) biases, emotional influences, personality dynamics, temperament and coping style. Studies have indicated that variation in genes can play an important role in the development of mental disorders, although the reliable identification of connections between specific genes and specific categories of disorder has proven more difficult. Environmental events surrounding pregnancy and birth have also been implicated. Traumatic brain injury may increase the risk of developing certain mental disorders. There have been some tentative inconsistent links found to certain viral infections, to substance misuse, and to general physical health.

Social influences have been found to be important, including abuse, neglect, bullying, social stress, traumatic events and other negative or overwhelming life experiences. The specific risks and pathways to particular disorders are less clear, however. Aspects of the wider community have also been implicated,^{[citation needed]} including employment problems, socioeconomic inequality, lack of social cohesion, problems linked to migration, and features of particular societies and cultures.

Theories

General theories

There are a number of theories or models seeking to explain the causes (etiology) of mental disorders. These theories may differ in regards to how they explain the cause of the disorder, how they treat the disorder, and their basic classification of mental disorders. There may also be differences in philosophy of mind regarding whether, or how, the mind is considered separately from the brain. 

During most of the 20th century, mental illness was believed to be caused by problematic relationships between children and their parents. This view was held well into the late 1990s, in which people still believed this child-parent relationship was a large determinant of severe mental illness, such as depression and schizophrenia. Today,  the belief is held that the child-parent relationship is of small importance in terms of causing mental illness compared to biological and genetic factors. So, the perceived causes of mental illness have changed over time and will most likely continue to alter while more research is done in this area.

Outside the West, community approaches remain a focus.

Medical or biomedical model

An overall distinction is also commonly made between a "medical model" (also known as a biomedical or disease model) and a "social model" (also known as an empowerment or recovery model) of mental disorder and disability, with the former focusing on hypothesized disease processes and symptoms, and the latter focusing on hypothesized social constructionism and social contexts.

Biological psychiatry has tended to follow a biomedical model focused on organic or "hardware" pathology of the brain, where many mental disorders are conceptualized as disorders of brain circuits likely caused by developmental processes shaped by a complex interplay of genetics and experience.

Biopsychosocial model

The primary model of contemporary mainstream Western psychiatry is the biopsychosocial model (BPS), which merges biological, psychological and social factors. For instance one view is that genetics accounts for 40% of a person's susceptibility to mental disorders while psychological and environmental factors account for the other 60%. It may be commonly neglected or misapplied in practice due to being too broad or relativistic, however. 

The most common view  is that disorders tend to result from genetic dispositions and environmental stressors, combining to cause patterns of distress or dysfunction or, more sharply, trigger disorders (Diathesis-stress model). A practical mixture of models may often be used to explain particular issues and disorders, although there may be difficulty defining boundaries for indistinct psychiatric syndromes.

Psychoanalytic theories

Psychoanalytic theories focus on unresolved internal and relational conflicts. These theories have been posited as overall explanations of mental disorder, although today most psychoanalytic groups are said to adhere to the biopsychosocial model and to accept an eclectic mix of subtypes of psychoanalysis. The psychoanalytic theory was originated by Sigmund Freud. This theory focuses on the impact of unconscious forces on human behavior. According to Freud, the personality is made up of three parts: the id, ego, and superego. The id operates under the pleasure principle, the ego operates under the reality principle, and the superego is the "conscience" and incorporates what is and is not socially acceptable into a person's value system. Also, according to the psychoanalytic theory, there are five stages of psycho-sexual development that everyone goes through: the oral stage, anal stage, phallic stage, latency stage, and genital stage. Mental disorders can be caused by an individual receiving too little or too much gratification in one of the psycho-sexual developmental stages. When this happens, the individual is said to be fixated in that developmental stage.

Attachment theory

Attachment theory is a kind of evolutionary-psychological approach sometimes applied in the context for mental disorders, which focuses on the role of early caregiver-child relationships, responses to danger, and the search for a satisfying reproductive relationship in adulthood. According to this theory, the more secure a child's attachment is to a nurturing adult, the more likely that child will maintain healthy relationships with others in their life. As found by the Strange Situation experiment run by Mary Ainsworth based on the formulations of John Bowlby, there are four main patterns of attachment: secure attachment, avoidant attachment, disorganized attachment and ambivalent attachment. These attachment patterns are found cross-culturally. Later research found a fourth pattern of attachment known as disorganized disoriented attachment. Secure attachments reflect trust in the child-caretaker relationship while insecure attachment reflects mistrust. The security of attachment in a child affects the child's emotional, cognitive, and social competence later in life.

Evolutionary psychology

Evolutionary psychology (or more specifically evolutionary psychopathology or psychiatry) has also been proposed as an overall theory, positing that many mental disorders involve the dysfunctional operation of mental modules adapted to ancestral physical or social environments but not necessarily to modern ones. Other theories suggest that mental illness could have evolutionary advantages for the species, including in enhancing creativity. Some related behavioral abnormalities have been found in non-human great apes. Evolutionary psychology applies Darwinian principles to human behavior by saying that human minds are products of natural selection and have specific functions. Humans strive to carry on their genetic legacy through their offspring. This theory identifies the environment as having a great effect on a person's mental development.

Factors affecting choice of models and theories

Psychiatrists may favour biomedical models because they believe such models make their discipline seem more esteemed. Similarly, families of mentally ill people tend to favour biomedical models because to do so gives less self-blame. If patients are seen by a more ethnically similar doctor, they are more likely to adopt a non-biomedical model.

Biological factors

Biological factors consist of anything physical that can cause adverse effects on a person's mental health. This includes genetics, prenatal damage, infections, exposure to toxins, brain defects or injuries, and substance abuse. Many professionals believe that the sole cause of mental disorders is based upon the biology of the brain and the nervous system.

Mind mentions genetic factors, long-term physical health conditions, and head injuries or epilepsy (affecting behaviour and mood) as factors that may possibly trigger an episode of mental illness.

Genetics

Family-linkage and twin studies have indicated that genetic factors often play an important role in the development of mental disorders. The reliable identification of specific genetic susceptibility to particular disorders, through linkage or association studies, has proven difficult. This has been reported to be likely due to the complexity of interactions between genes, environmental events, and early development or to the need for new research strategies. The heritability of behavioral traits associated with mental disorder may be greater in permissive than in restrictive environments, and susceptibility genes probably work through both "within-the-skin" (physiological) pathways and "outside-the-skin" (behavioral and social) pathways. Investigations increasingly focus on links between genes and endophenotypes—more specific traits (including neurophysiological, biochemical, endocrinological, neuroanatomical, cognitive, or neuropsychological)—rather than disease categories. With regard to a prominent mental disorder, schizophrenia, for a long time consensus among scientists was that certain alleles (forms of genes) were responsible for schizophrenia, but some research has indicated only multiple, rare mutations thought to alter neurodevelopmental pathways that can ultimately contribute to schizophrenia; virtually every rare structural mutation was different in each individual.

Research has shown that many conditions are polygenic meaning there are multiple defective genes rather than only one that are responsible for a disorder. Schizophrenia and Alzheimer's are both examples of hereditary mental disorders.

The increasing understanding of brain plasticity (neuroplasticity) raises questions of whether some brain differences may be caused by mental illnesses, rather than pre-existing and causing them.

Prenatal damage

Any damage that occurs to a fetus while still in its mother's womb is considered prenatal damage. If the pregnant mother uses drugs or alcohol or is exposed to illnesses or infections then mental disorders can develop in the fetus. According to research, certain conditions, such as autism result from a disruption of early fetal brain progression.

Environmental events surrounding pregnancy and birth have been linked to an increased development of mental illness in the offspring. This includes maternal exposure to serious psychological stress or trauma, conditions of famine, obstetric birth complications, infections, and gestational exposure to alcohol or cocaine. Such factors have been hypothesized to affect specific areas of neurodevelopment within the general developmental context and to restrict neuroplasticity.

Infection, disease and toxins

A number of psychiatric disorders have often been tentatively linked with microbial pathogens, particularly viruses; however while there have been some suggestions of links from animal studies, and some inconsistent evidence for infectious and immune mechanisms (including prenatally) in some human disorders, infectious disease models in psychiatry are reported to have not yet shown significant promise except in isolated cases.

There have been some inconsistent findings of links between infection by the parasite Toxoplasma gondii and human mental disorders such as schizophrenia, with the direction of causality unclear. A number of diseases of the white matter can cause symptoms of mental disorder.

Poorer general health has been found among individuals with severe mental illnesses, thought to be due to direct and indirect factors including diet, bacterial infections, substance use, exercise levels, effects of medications, socioeconomic disadvantages, lowered help-seeking or treatment adherence, or poorer healthcare provision. Some chronic general medical conditions have been linked to some aspects of mental disorder, such as AIDS-related psychosis. 

The current research on Lyme's disease caused by a deer tick, and related toxins, is expanding the link between bacterial infections and mental illness.

Research shows that infections and exposure to toxins such as HIV and streptococcus cause dementia and OCD respectively. The infections or toxins trigger a change in the brain chemistry, which can develop into a mental disorder.

Injury and brain defects

Any damage to the brain can cause a mental disorder. The brain is the control system for the nervous system and the rest of the body. Without it the body cannot function properly.

Higher rates of mood, psychotic, and substance abuse disorders have been found following traumatic brain injury (TBI). Findings on the relationship between TBI severity and prevalence of subsequent psychiatric disorders have been inconsistent, and occurrence has been linked to prior mental health problems as well as direct neurophysiological effects, in a complex interaction with personality and attitude and social influences.

Head trauma is classified as either open or closed head injury. In open head injury the skull is penetrated and brain tissue is destroyed in a localized area. Closed head injury is more common, the skull is not penetrated but there is an impact of the brain against the skull which can create permanent structural damage (e.g. subdural hematoma). With both types, symptoms may disappear or persist over time. It has been found that typically the longer the length of time spent unconscious and the length of post-traumatic amnesia the worse the prognosis for the individual. The cognitive residual symptoms of head trauma are associated with the type of injury (either open head injury or closed head injury)and the amount of tissue destroyed. Symptoms of closed injury head trauma tend to be the experience of intellectual deficits in abstract reasoning ability, judgement, and memory, and also marked personality changes. Symptoms of open injury head trauma tend to be the experience of classic neuropsychological syndromes like aphasia, visual-spatial disorders, and types of memory or perceptual disorders.

Brain tumors are classified as either malignant and benign, and as intrinsic (directly infiltrate the parenchyma of the brain) or extrinsic (grows on the external surface of the brain and produces symptoms as a result of pressure on the brain tissue). Progressive cognitive changes associated with brain tumors may include confusion, poor comprehension, and even dementia. Symptoms tend to depend on the location of the tumor on the brain. For example, tumors on the frontal lobe tend to be associated with the symptoms of impairment of judgment, apathy, and loss of the ability to regulate/modulate behavior.

Findings have indicated abnormal functioning of brainstem structures in individuals with mental disorders such as schizophrenia, and other disorders that have to do with impairments in maintaining sustained attention. Some abnormalities in the average size or shape of some regions of the brain have been found in some disorders, reflecting genes and/or experience. Studies of schizophrenia have tended to find enlarged ventricles and sometimes reduced volume of the cerebrum and hippocampus, while studies of (psychotic) bipolar disorder have sometimes found increased amygdala volume. Findings differ over whether volumetric abnormalities are risk factors or are only found alongside the course of mental health problems, possibly reflecting neurocognitive or emotional stress processes and/or medication use or substance use. Some studies have also found reduced hippocampal volumes in major depression, possibly worsening with time depressed.

Neurotransmitter systems

Abnormal levels of dopamine activity have been correlated with a number of disorders (e.g., reduced in ADHD and OCD, and increased in schizophrenia). Dysfunction in serotonin and other monoamine neurotransmitters (e.g., norepinephrine and dopamine), and their associated neural networks, are also moderately correlated with certain mental disorders, including major depression, obsessive compulsive disorder, phobias, posttraumatic stress disorder, and generalized anxiety disorder. Studies of depleted levels of monoamine neurotransmitters show an association with depression and other psychiatric disorders, but "... it should be questioned whether 5-HT [serotonin] represents just one of the final, and not the main, factors in the neurological chain of events underlying psychopathological symptoms...." 

Simplistic "chemical imbalance" explanations for mental disorders have never received empirical support; and most prominent psychiatrists, neuroscientists, and psychologists have not espoused such ill-defined, facile etiological theories. Instead, neurotransmitter systems have been understood in the context of the diathesis-stress or biopsychosocial models. The following 1967 quote from renowned psychiatric and neuroscience researchers exemplifies this more sophisticated understanding (in contrast to the woolly "chemical imbalance" notion).

Whereas specific genetic factors may be of importance in the etiology of some, and possibly all, depressions, it is equally conceivable that early experiences of the infant or child may cause enduring biochemical changes, and that these may predispose some individuals to depressions in adulthood. It is not likely that changes in the metabolism of the biogenic amines alone will account for the complex phenomena of normal or pathological affect.

Substance abuse

Substance abuse, especially long-term abuse, can cause or exacerbate many mental disorders. Alcoholism is linked to depression while abuse of amphetamines and LSD can leave a person feeling paranoid and anxious.

Correlations of mental disorders with drug use include cannabis, alcohol and caffeine. Caffeine use is correlated with anxiety and suicide. Illicit drugs have the ability to stimulate particular parts of the brain which can affect development in adolescence. Cannabis has been found to worsen depression and lessen an individual's motivation. Alcohol has the potential to damage "white matter" in the brain which affects thinking and memory. Alcohol has been found to be a serious problem in many countries due to many people participating in excessive drinking or binge drinking.

Life experience and environmental factors

The term "environment" is very loosely defined when it comes to mental illness. Unlike biological and psychological causes, environmental causes are stressors that individuals deal with in everyday life. These stressors range from financial issues to having low self-esteem. Environmental causes are more psychologically based thus making them more closely related. Events that evoke feelings of loss or damage are most likely to cause a mental disorder to develop in an individual. Environmental factors include but are not limited a dysfunctional home life, poor relationships with others, substance abuse, not meeting social expectations, low self-esteem and poverty.

Mind mentions childhood abuse, trauma, violence or neglect, social isolation, loneliness or discrimination, the death of someone close, stress, homelessness or poor housing, social disadvantage, poverty or debt, unemployment, caring for a family member or friend, significant trauma as an adult, such as military combat, and being involved in a serious accident or being the victim of a violent crime as possibly triggering an episode of mental illness.

Repeating generational patterns have been found to be a risk factor for mental illness.

Life events and emotional stress

It is reported that treatment in childhood and in adulthood, including sexual abuse, physical abuse, emotional abuse, domestic violence and bullying, has been linked to the development of mental disorders, through a complex interaction of societal, family, psychological and biological factors. Negative or stressful life events more generally have been implicated in the development of a range of disorders, including mood and anxiety disorders. The main risks appear to be from a cumulative combination of such experiences over time, although exposure to a single major trauma can sometimes lead to psychopathology, including PTSD. Resilience to such experiences varies, and a person may be resistant to some forms of experience but susceptible to others. Features associated with variations in resilience include genetic vulnerability, temperamental characteristics, cognitive set, coping patterns, and other experiences.

For bipolar disorder, stress (such as childhood adversity) is not a specific cause, but does place genetically and biologically vulnerable individuals at risk for a more severe course of illness.

Poor parenting, abuse and neglect

Poor parenting has been found to be a risk factor for depression and anxiety. Separation or bereavement in families, and childhood trauma, have been found to be risk factors for psychosis and schizophrenia.

Severe psychological trauma such as abuse can wreak havoc on a person's life. Children are much more susceptible to psychological harm from traumatic events than adults. Once again, the reaction to the trauma will vary based on the person as well as the individual's age. The impact of these events is influenced by several factors: the type of event, the length of exposure the individual had to the event, and the extent to which the individual and their family/friends were personally affected by the event. Human-caused disasters, such as a tumultuous childhood affect children more than natural disasters.

Neglect is a type of maltreatment related to the failure to provide needed, age-appropriate care, supervision and protection. It is not to be confused with abuse, which, in this context, is defined as any action that intentionally harms or injures another person. Neglect most often happens during childhood by the parents or caretakers. Oftentimes, parents who are guilty of neglect were also neglected as children. The long-term effects of neglect are reduced physical, emotional, and mental health in a child and throughout adulthood.

The Adverse Childhood Experiences Study

Adverse childhood experiences (ACEs) are various forms of maltreatment and household dysfunction experienced in childhood. The Adverse Childhood Experiences Study has shown a strong dose–response relationship between ACEs and numerous health, social, and behavioral problems throughout a person's lifespan, including suicide attempts and frequency of depressive episodes. Children's neurological development can be disrupted when they are chronically exposed to stressful events such as physical, emotional, or sexual abuse, physical or emotional neglect, witnessing violence in the household, or a parent being incarcerated or suffering from a mental illness. As a result, the child's cognitive functioning or ability to cope with negative or disruptive emotions may be impaired. Over time, the child may adopt various harmful coping strategies that can contribute to later disease and disability.

Relationships

Relationship issues have been consistently linked to the development of mental disorders, with continuing debate on the relative impact of the home environment or work/school and peer groups. Issues with parenting skills or parental depression or other problems may be a risk factor. Parental divorce appears to increase risk, perhaps only if there is family discord or disorganization, although a warm supportive relationship with one parent may compensate. Details of infant feeding, weaning, toilet training etc. do not appear to be importantly linked to psychopathology. Early social privation, or lack of ongoing, harmonious, secure, committed relationships, have been implicated in the development of mental disorders.

Some approaches, such as certain theories of co-counseling, may see all non-neurological mental disorders as the result of the self-regulating mechanisms of the mind (which accompany the physical expression of emotions) not being allowed to operate. 

How an individual interacts with others as well as the quality of relationships can greatly increase or decrease a person's quality of living. Continuous fighting with friends and family can all lead to an increased risk of developing a mental illness. A dysfunctional family may include disobedience, child neglect and/or abuse which occurs regularly. These types of families are often a product of an unhealthy co-dependent relationship on the part of the head of the household (usually to drugs). 

Losing a loved one, especially at an early age can have lasting effects on an individual. The individual may feel fear, guilt, anger or loneliness. This can drive a person into solitude and depression. They may turn to alcohol and drugs to cope with their feelings. 

Divorce is also another factor that can take a toll on both children and adults alike. Divorcees may suffer from emotional adjustment problems due to a loss of intimacy and social connections. Newer statistics show that the negative effects of divorce have been greatly exaggerated. The effects of divorce in children are based on three main factors: the quality of their relationship with each of their parents before the separation, the intensity and duration of the parental conflict, and the parents' ability to focus on the needs of children in their divorce.

Social expectations and esteem

How individuals view themselves ultimately determines who they are, their abilities and what they can be. Having both too low of self-esteem as well as too high of one can be detrimental to an individual's mental health. A person's self-esteem plays a much larger role in their overall happiness and quality of life. Poor self-esteem whether it be too high or too low can result in aggression, violence, self-deprecating behavior, anxiety, and other mental disorders. 

Not fitting in with the masses can result in bullying and other types of emotional abuse. Bullying can result in depression, feelings of anger, loneliness.

Poverty

Studies show that there is a direct correlation between poverty and mental illness. The lower the socioeconomic status of an individual the higher the risk of mental illness. Impoverished people are actually two to three times more likely to develop mental illness than those of a higher economic class. 

Low levels of self-efficiency and self-worth are commonly experienced by children of disadvantaged families or those from the economic underclass. Theorists of child development have argued that persistent poverty leads to high levels of psychopathology and poor self-concepts.

This increased risk for psychiatric complications remains consistent for all individuals among the impoverished population, regardless of any in-group demographic differences that they may possess. These families must deal with economic stressors like unemployment and lack of affordable housing, which can lead to mental health disorders. A person's socioeconomic class outlines the psychosocial, environmental, behavioral, and biomedical risk factors that are associated with mental health.

According to findings there is a strong association between poverty and substance abuse. Substance abuse only perpetuates a continuous cycle. It can make it extremely difficult for individuals to find and keep jobs. As stated earlier, both financial problems and substance abuse can cause mental illnesses to develop.

Communities and cultures

Mental disorders have been linked to the overarching social, economic and cultural system. Some non-Western views take this community approach.

Problems in communities or cultures, including poverty, unemployment or underemployment, lack of social cohesion, and migration, have been associated with the development of mental disorders. Stresses and strains related to socioeconomic position (socioeconomic status (SES) or social class) have been linked to the occurrence of major mental disorders, with a lower or more insecure educational, occupational, economic or social position generally linked to more mental disorders. There have been mixed findings on the nature of the links and on the extent to which pre-existing personal characteristics influence the links. Both personal resources and community factors have been implicated, as well as interactions between individual-level and regional-level income levels. The causal role of different socioeconomic factors may vary by country. Socioeconomic deprivation in neighborhoods can cause worse mental health, even after accounting for genetic factors. In addition, minority ethnic groups, including first or second-generation immigrants, have been found to be at greater risk for developing mental disorders, which has been attributed to various kinds of life insecurities and disadvantages, including racism. The direction of causality is sometimes unclear, and alternative hypotheses such as the Drift Hypothesis sometimes need to be discounted.

Psychological and individual factors, including resilience

Some clinicians believe that psychological characteristics alone determine mental disorders. Others speculate that abnormal behavior can be explained by a mix of social and psychological factors. In many examples, environmental and psychological triggers complement one another resulting in emotional stress, which in turn activates a mental illness Each person is unique in how they will react to psychological stressors. What may break one person may have little to no effect on another. Psychological stressors, which can trigger mental illness, are as follows: emotional, physical or sexual abuse, loss of a significant loved one, neglect and being unable to relate to others.

The inability to relate to others is also known as emotional detachment. Emotional detachment makes it difficult for an individual to empathize with others or to share their own feelings. An emotionally detached person may try to rationalize or apply logic to a situation to which there is no logical explanation. These individuals tend to stress the importance of their independence and may be a bit neurotic. Oftentimes, the inability to relate to others stems from a traumatic event. 

Mental characteristics of individuals, as assessed by both neurological and psychological studies, have been linked to the development and maintenance of mental disorders. This includes cognitive or neurocognitive factors, such as the way a person perceives, thinks or feels about certain things; or an individual's overall personality, temperament or coping style or the extent of protective factors or "positive illusions" such as optimism, personal control and a sense of meaning.

Reason

From Wikipedia, the free encyclopedia

 

Reason is the capacity of consciously making sense of things, establishing and verifying facts, applying logic, and changing or justifying practices, institutions, and beliefs based on new or existing information. It is closely associated with such characteristically human activities as philosophy, science, language, mathematics and art, and is normally considered to be a distinguishing ability possessed by humans. Reason, or an aspect of it, is sometimes referred to as rationality. 

 

Reasoning is associated with thinking, cognition, and intellect. The philosophical field of logic studies ways in which humans reason formally through argument. Reasoning may be subdivided into forms of logical reasoning (forms associated with the strict sense): deductive reasoning, inductive reasoning, abductive reasoning; and other modes of reasoning considered more informal, such as intuitive reasoning and verbal reasoning. Along these lines, a distinction is often drawn between logical, discursive reasoning (reason proper), and intuitive reasoning, in which the reasoning process through intuition—however valid—may tend toward the personal and the subjectively opaque. In some social and political settings logical and intuitive modes of reasoning may clash, while in other contexts intuition and formal reason are seen as complementary rather than adversarial. For example, in mathematics, intuition is often necessary for the creative processes involved with arriving at a formal proof, arguably the most difficult of formal reasoning tasks.

Reasoning, like habit or intuition, is one of the ways by which thinking moves from one idea to a related idea. For example, reasoning is the means by which rational individuals understand sensory information from their environments, or conceptualize abstract dichotomies such as cause and effect, truth and falsehood, or ideas regarding notions of good or evil. Reasoning, as a part of executive decision making, is also closely identified with the ability to self-consciously change, in terms of goals, beliefs, attitudes, traditions, and institutions, and therefore with the capacity for freedom and self-determination.

In contrast to the use of "reason" as an abstract noun, a reason is a consideration given which either explains or justifies events, phenomena, or behavior. Reasons justify decisions, reasons support explanations of natural phenomena; reasons can be given to explain the actions (conduct) of individuals.

Using reason, or reasoning, can also be described more plainly as providing good, or the best, reasons. For example, when evaluating a moral decision, "morality is, at the very least, the effort to guide one's conduct by reason—that is, doing what there are the best reasons for doing—while giving equal [and impartial] weight to the interests of all those affected by what one does."

Psychologists and cognitive scientists have attempted to study and explain how people reason, e.g. which cognitive and neural processes are engaged, and how cultural factors affect the inferences that people draw. The field of automated reasoning studies how reasoning may or may not be modeled computationally. Animal psychology considers the question of whether animals other than humans can reason.

Etymology and related words

In the English language and other modern European languages, "reason", and related words, represent words which have always been used to translate Latin and classical Greek terms in the sense of their philosophical usage.

• The original Greek term was "λόγος" logos, the root of the modern English word "logic" but also a word which could mean for example "speech" or "explanation" or an "account" (of money handled).
• As a philosophical term logos was translated in its non-linguistic senses in Latin as ratio. This was originally not just a translation used for philosophy, but was also commonly a translation for logos in the sense of an account of money.
• French raison is derived directly from Latin, and this is the direct source of the English word "reason".

The earliest major philosophers to publish in English, such as Francis Bacon, Thomas Hobbes, and John Locke also routinely wrote in Latin and French, and compared their terms to Greek, treating the words "logos", "ratio", "raison" and "reason" as interchangeable. The meaning of the word "reason" in senses such as "human reason" also overlaps to a large extent with "rationality" and the adjective of "reason" in philosophical contexts is normally "rational", rather than "reasoned" or "reasonable". Some philosophers, Thomas Hobbes for example, also used the word ratiocination as a synonym for "reasoning".

Philosophical history

Francisco de Goya, The Sleep of Reason Produces Monsters (El sueño de la razón produce monstruos), c. 1797

 

The proposal that reason gives humanity a special position in nature has been argued to be a defining characteristic of western philosophy and later western modern science, starting with classical Greece. Philosophy can be described as a way of life based upon reason, and in the other direction reason has been one of the major subjects of philosophical discussion since ancient times. Reason is often said to be reflexive, or "self-correcting", and the critique of reason has been a persistent theme in philosophy. It has been defined in different ways, at different times, by different thinkers about human nature.

Classical philosophy

For many classical philosophers, nature was understood teleologically, meaning that every type of thing had a definitive purpose which fit within a natural order that was itself understood to have aims. Perhaps starting with Pythagoras or Heraclitus, the cosmos is even said to have reason. Reason, by this account, is not just one characteristic that humans happen to have, and that influences happiness amongst other characteristics. Reason was considered of higher stature than other characteristics of human nature, such as sociability, because it is something humans share with nature itself, linking an apparently immortal part of the human mind with the divine order of the cosmos itself. Within the human mind or soul (psyche), reason was described by Plato as being the natural monarch which should rule over the other parts, such as spiritedness (thumos) and the passions. Aristotle, Plato's student, defined human beings as rational animals, emphasizing reason as a characteristic of human nature. He defined the highest human happiness or well being (eudaimonia) as a life which is lived consistently, excellently and completely in accordance with reason.

The conclusions to be drawn from the discussions of Aristotle and Plato on this matter are amongst the most debated in the history of philosophy. But teleological accounts such as Aristotle's were highly influential for those who attempt to explain reason in a way which is consistent with monotheism and the immortality and divinity of the human soul. For example, in the neo-platonist account of Plotinus, the cosmos has one soul, which is the seat of all reason, and the souls of all individual humans are part of this soul. Reason is for Plotinus both the provider of form to material things, and the light which brings individuals souls back into line with their source. Such neo-Platonist accounts of the rational part of the human soul were standard amongst medieval Islamic philosophers, and under this influence, mainly via Averroes, came to be debated seriously in Europe until well into the renaissance, and they remain important in Iranian philosophy.

Subject-centred reason in early modern philosophy

The early modern era was marked by a number of significant changes in the understanding of reason, starting in Europe. One of the most important of these changes involved a change in the metaphysical understanding of human beings. Scientists and philosophers began to question the teleological understanding of the world. Nature was no longer assumed to be human-like, with its own aims or reason, and human nature was no longer assumed to work according to anything other than the same "laws of nature" which affect inanimate things. This new understanding eventually displaced the previous world view that derived from a spiritual understanding of the universe. 

René Descartes

 

Accordingly, in the 17th century, René Descartes explicitly rejected the traditional notion of humans as "rational animals", suggesting instead that they are nothing more than "thinking things" along the lines of other "things" in nature. Any grounds of knowledge outside that understanding was, therefore, subject to doubt.

In his search for a foundation of all possible knowledge, Descartes deliberately decided to throw into doubt all knowledge – except that of the mind itself in the process of thinking:

At this time I admit nothing that is not necessarily true. I am therefore precisely nothing but a thinking thing; that is a mind, or intellect, or understanding, or reason – words of whose meanings I was previously ignorant.

This eventually became known as epistemological or "subject-centred" reason, because it is based on the knowing subject, who perceives the rest of the world and itself as a set of objects to be studied, and successfully mastered by applying the knowledge accumulated through such study. Breaking with tradition and many thinkers after him, Descartes explicitly did not divide the incorporeal soul into parts, such as reason and intellect, describing them as one indivisible incorporeal entity. 

A contemporary of Descartes, Thomas Hobbes described reason as a broader version of "addition and subtraction" which is not limited to numbers. This understanding of reason is sometimes termed "calculative" reason. Similar to Descartes, Hobbes asserted that "No discourse whatsoever, can end in absolute knowledge of fact, past, or to come" but that "sense and memory" is absolute knowledge.

In the late 17th century, through the 18th century, John Locke and David Hume developed Descartes' line of thought still further. Hume took it in an especially skeptical direction, proposing that there could be no possibility of deducing relationships of cause and effect, and therefore no knowledge is based on reasoning alone, even if it seems otherwise.

Hume famously remarked that, "We speak not strictly and philosophically when we talk of the combat of passion and of reason. Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them." Hume also took his definition of reason to unorthodox extremes by arguing, unlike his predecessors, that human reason is not qualitatively different from either simply conceiving individual ideas, or from judgments associating two ideas, and that "reason is nothing but a wonderful and unintelligible instinct in our souls, which carries us along a certain train of ideas, and endows them with particular qualities, according to their particular situations and relations." It followed from this that animals have reason, only much less complex than human reason. 

In the 18th century, Immanuel Kant attempted to show that Hume was wrong by demonstrating that a "transcendental" self, or "I", was a necessary condition of all experience. Therefore, suggested Kant, on the basis of such a self, it is in fact possible to reason both about the conditions and limits of human knowledge. And so long as these limits are respected, reason can be the vehicle of morality, justice, aesthetics, theories of knowledge (epistemology), and understanding.

Substantive and formal reason

In the formulation of Kant, who wrote some of the most influential modern treatises on the subject, the great achievement of reason (German: Vernunft) is that it is able to exercise a kind of universal law-making. Kant was able therefore to reformulate the basis of moral-practical, theoretical and aesthetic reasoning, on "universal" laws. 

Here practical reasoning is the self-legislating or self-governing formulation of universal norms, and theoretical reasoning the way humans posit universal laws of nature.

Under practical reason, the moral autonomy or freedom of human beings depends on their ability to behave according to laws that are given to them by the proper exercise of that reason. This contrasted with earlier forms of morality, which depended on religious understanding and interpretation, or nature for their substance.

According to Kant, in a free society each individual must be able to pursue their goals however they see fit, so long as their actions conform to principles given by reason. He formulated such a principle, called the "categorical imperative", which would justify an action only if it could be universalized:

Act only according to that maxim whereby you can, at the same time, will that it should become a universal law.

In contrast to Hume then, Kant insists that reason itself (German Vernunft) has natural ends itself, the solution to the metaphysical problems, especially the discovery of the foundations of morality. Kant claimed that this problem could be solved with his "transcendental logic" which unlike normal logic is not just an instrument, which can be used indifferently, as it was for Aristotle, but a theoretical science in its own right and the basis of all the others.

According to Jürgen Habermas, the "substantive unity" of reason has dissolved in modern times, such that it can no longer answer the question "How should I live?" Instead, the unity of reason has to be strictly formal, or "procedural." He thus described reason as a group of three autonomous spheres (on the model of Kant's three critiques):

1. Cognitive–instrumental reason is the kind of reason employed by the sciences. It is used to observe events, to predict and control outcomes, and to intervene in the world on the basis of its hypotheses;
2. Moral–practical reason is what we use to deliberate and discuss issues in the moral and political realm, according to universalizable procedures (similar to Kant's categorical imperative); and
3. Aesthetic reason is typically found in works of art and literature, and encompasses the novel ways of seeing the world and interpreting things that those practices embody.

For Habermas, these three spheres are the domain of experts, and therefore need to be mediated with the "lifeworld" by philosophers. In drawing such a picture of reason, Habermas hoped to demonstrate that the substantive unity of reason, which in pre-modern societies had been able to answer questions about the good life, could be made up for by the unity of reason's formalizable procedures.

The critique of reason

Hamann, Herder, Kant, Hegel, Kierkegaard, Nietzsche, Heidegger, Foucault, Rorty, and many other philosophers have contributed to a debate about what reason means, or ought to mean. Some, like Kierkegaard, Nietzsche, and Rorty, are skeptical about subject-centred, universal, or instrumental reason, and even skeptical toward reason as a whole. Others, including Hegel, believe that it has obscured the importance of intersubjectivity, or "spirit" in human life, and attempt to reconstruct a model of what reason should be. 

Some thinkers, e.g. Foucault, believe there are other forms of reason, neglected but essential to modern life, and to our understanding of what it means to live a life according to reason.

In the last several decades, a number of proposals have been made to "re-orient" this critique of reason, or to recognize the "other voices" or "new departments" of reason.

For example, in opposition to subject-centred reason, Habermas has proposed a model of communicative reason that sees it as an essentially cooperative activity, based on the fact of linguistic intersubjectivity.

Nikolas Kompridis has proposed a widely encompassing view of reason as "that ensemble of practices that contributes to the opening and preserving of openness" in human affairs, and a focus on reason's possibilities for social change.

The philosopher Charles Taylor, influenced by the 20th century German philosopher Martin Heidegger, has proposed that reason ought to include the faculty of disclosure, which is tied to the way we make sense of things in everyday life, as a new "department" of reason.

In the essay "What is Enlightenment?", Michel Foucault proposed a concept of critique based on Kant's distinction between "private" and "public" uses of reason. This distinction, as suggested, has two dimensions:

• Private reason is the reason that is used when an individual is "a cog in a machine" or when one "has a role to play in society and jobs to do: to be a soldier, to have taxes to pay, to be in charge of a parish, to be a civil servant."
• Public reason is the reason used "when one is reasoning as a reasonable being (and not as a cog in a machine), when one is reasoning as a member of reasonable humanity." In these circumstances, "the use of reason must be free and public."

Reason compared to related concepts

Compared to logic

The terms "logic" or "logical" are sometimes used as if they were identical with the term "reason" or with the concept of being "rational", or sometimes logic is seen as the most pure or the defining form of reason. For example in modern economics, rational choice is assumed to equate to logically consistent choice. 

Reason and logic can however be thought of as distinct, although logic is one important aspect of reason. Author Douglas Hofstadter, in Gödel, Escher, Bach, characterizes the distinction in this way. Logic is done inside a system while reason is done outside the system by such methods as skipping steps, working backward, drawing diagrams, looking at examples, or seeing what happens if you change the rules of the system.

Reason is a type of thought, and the word "logic" involves the attempt to describe rules or norms by which reasoning operates, so that orderly reasoning can be taught. The oldest surviving writing to explicitly consider the rules by which reason operates are the works of the Greek philosopher Aristotle, especially Prior Analysis and Posterior Analysis. Although the Ancient Greeks had no separate word for logic as distinct from language and reason, Aristotle's newly coined word "syllogism" (syllogismos) identified logic clearly for the first time as a distinct field of study. When Aristotle referred to "the logical" (hē logikē), he was referring more broadly to rational thought.

Reason compared to cause-and-effect thinking, and symbolic thinking

As pointed out by philosophers such as Hobbes, Locke and Hume, some animals are also clearly capable of a type of "associative thinking", even to the extent of associating causes and effects. A dog once kicked, can learn how to recognize the warning signs and avoid being kicked in the future, but this does not mean the dog has reason in any strict sense of the word. It also does not mean that humans acting on the basis of experience or habit are using their reason.

Human reason requires more than being able to associate two ideas, even if those two ideas might be described by a reasoning human as a cause and an effect, perceptions of smoke, for example, and memories of fire. For reason to be involved, the association of smoke and the fire would have to be thought through in a way which can be explained, for example as cause and effect. In the explanation of Locke, for example, reason requires the mental use of a third idea in order to make this comparison by use of syllogism.

More generally, reason in the strict sense requires the ability to create and manipulate a system of symbols, as well as indices and icons, according to Charles Sanders Peirce, the symbols having only a nominal, though habitual, connection to either smoke or fire. One example of such a system of artificial symbols and signs is language.

The connection of reason to symbolic thinking has been expressed in different ways by philosophers. Thomas Hobbes described the creation of "Markes, or Notes of remembrance" (Leviathan Ch. 4) as speech. He used the word speech as an English version of the Greek word logos so that speech did not need to be communicated. When communicated, such speech becomes language, and the marks or notes or remembrance are called "Signes" by Hobbes. Going further back, although Aristotle is a source of the idea that only humans have reason (logos), he does mention that animals with imagination, for whom sense perceptions can persist, come closest to having something like reasoning and nous, and even uses the word "logos" in one place to describe the distinctions which animals can perceive in such cases.

Reason, imagination, mimesis, and memory

Reason and imagination rely on similar mental processes. Imagination is not only found in humans. Aristotle, for example, stated that phantasia (imagination: that which can hold images or phantasmata) and phronein (a type of thinking that can judge and understand in some sense) also exist in some animals. According to him, both are related to the primary perceptive ability of animals, which gathers the perceptions of different senses and defines the order of the things that are perceived without distinguishing universals, and without deliberation or logos. But this is not yet reason, because human imagination is different. 

The recent modern writings of Terrence Deacon and Merlin Donald, writing about the origin of language, also connect reason connected to not only language, but also mimesis, More specifically they describe the ability to create language as part of an internal modeling of reality specific to humankind. Other results are consciousness, and imagination or fantasy. In contrast, modern proponents of a genetic predisposition to language itself include Noam Chomsky and Steven Pinker, to whom Donald and Deacon can be contrasted. 

As reason is symbolic thinking, and peculiarly human, then this implies that humans have a special ability to maintain a clear consciousness of the distinctness of "icons" or images and the real things they represent. Starting with a modern author, Merlin Donald writes

A dog might perceive the "meaning" of a fight that was realistically play-acted by humans, but it could not reconstruct the message or distinguish the representation from its referent (a real fight). [...] Trained apes are able to make this distinction; young children make this distinction early – hence, their effortless distinction between play-acting an event and the event itself.

In classical descriptions, an equivalent description of this mental faculty is eikasia, in the philosophy of Plato. This is the ability to perceive whether a perception is an image of something else, related somehow but not the same, and therefore allows humans to perceive that a dream or memory or a reflection in a mirror is not reality as such. What Klein refers to as dianoetic eikasia is the eikasia concerned specifically with thinking and mental images, such as those mental symbols, icons, signes, and marks discussed above as definitive of reason. Explaining reason from this direction: human thinking is special in the way that we often understand visible things as if they were themselves images of our intelligible "objects of thought" as "foundations" (hypothēses in Ancient Greek). This thinking (dianoia) is "...an activity which consists in making the vast and diffuse jungle of the visible world depend on a plurality of more 'precise' noēta."

Both Merlin Donald and the Socratic authors such as Plato and Aristotle emphasize the importance of mimesis, often translated as imitation or representation. Donald writes

Imitation is found especially in monkeys and apes [... but ...] Mimesis is fundamentally different from imitation and mimicry in that it involves the invention of intentional representations. [...] Mimesis is not absolutely tied to external communication.

Mimēsis is a concept, now popular again in academic discussion, that was particularly prevalent in Plato's works, and within Aristotle, it is discussed mainly in the Poetics. In Michael Davis's account of the theory of man in this work.

It is the distinctive feature of human action, that whenever we choose what we do, we imagine an action for ourselves as though we were inspecting it from the outside. Intentions are nothing more than imagined actions, internalizings of the external. All action is therefore imitation of action; it is poetic...

Donald like Plato (and Aristotle, especially in On Memory and Recollection), emphasizes the peculiarity in humans of voluntary initiation of a search through one's mental world. The ancient Greek anamnēsis, normally translated as "recollection" was opposed to mneme or memory. Memory, shared with some animals, requires a consciousness not only of what happened in the past, but also that something happened in the past, which is in other words a kind of eikasia "...but nothing except man is able to recollect." Recollection is a deliberate effort to search for and recapture something once known. Klein writes that, "To become aware of our having forgotten something means to begin recollecting." Donald calls the same thing autocueing, which he explains as follows: "Mimetic acts are reproducible on the basis of internal, self-generated cues. This permits voluntary recall of mimetic representations, without the aid of external cues – probably the earliest form of representational thinking." 

In a celebrated paper in modern times, the fantasy author and philologist J.R.R. Tolkien wrote in his essay "On Fairy Stories" that the terms "fantasy" and "enchantment" are connected to not only "....the satisfaction of certain primordial human desires...." but also "...the origin of language and of the mind."

Logical reasoning methods and argumentation

Looking at logical categorizations of different types of reasoning the traditional main division made in philosophy is between deductive reasoning and inductive reasoning. Formal logic has been described as the science of deduction. The study of inductive reasoning is generally carried out within the field known as informal logic or critical thinking.

Deductive reasoning

A subdivision of Philosophy is Logic. Logic is the study of reasoning. Deduction is a form of reasoning in which a conclusion follows necessarily from the stated premises. A deduction is also the conclusion reached by a deductive reasoning process. One classic example of deductive reasoning is that found in syllogisms like the following:

• Premise 1: All humans are mortal.
• Premise 2: Socrates is a human.
• Conclusion: Socrates is mortal.

The reasoning in this argument is valid, because there is no way in which the premises, 1 and 2, could be true and the conclusion, 3, be false.

Inductive reasoning

Induction is a form of inference producing propositions about unobserved objects or types, either specifically or generally, based on previous observation. It is used to ascribe properties or relations to objects or types based on previous observations or experiences, or to formulate general statements or laws based on limited observations of recurring phenomenal patterns. 

Inductive reasoning contrasts strongly with deductive reasoning in that, even in the best, or strongest, cases of inductive reasoning, the truth of the premises does not guarantee the truth of the conclusion. Instead, the conclusion of an inductive argument follows with some degree of probability. Relatedly, the conclusion of an inductive argument contains more information than is already contained in the premises. Thus, this method of reasoning is ampliative. 

A classic example of inductive reasoning comes from the empiricist David Hume:

• Premise: The sun has risen in the east every morning up until now.
• Conclusion: The sun will also rise in the east tomorrow.

Analogical reasoning

Analogical reasoning is a form of inductive reasoning from a particular to a particular. It is often used in case-based reasoning, especially legal reasoning. An example follows:

• Premise 1: Socrates is human and mortal.
• Premise 2: Plato is human.
• Conclusion: Plato is mortal.

Analogical reasoning is a weaker form of inductive reasoning from a single example, because inductive reasoning typically uses a large number of examples to reason from the particular to the general. Analogical reasoning often leads to wrong conclusions. For example:

• Premise 1: Socrates is human and male.
• Premise 2: Ada Lovelace is human.
• Conclusion: Therefore Ada Lovelace is male.

Abductive reasoning

Abductive reasoning, or argument to the best explanation, is a form of reasoning that doesn't fit in deductive or inductive, since it starts with incomplete set of observations and proceeds with likely possible explanations so the conclusion in an abductive argument does not follow with certainty from its premises and concerns something unobserved. What distinguishes abduction from the other forms of reasoning is an attempt to favour one conclusion above others, by subjective judgement or attempting to falsify alternative explanations or by demonstrating the likelihood of the favoured conclusion, given a set of more or less disputable assumptions. For example, when a patient displays certain symptoms, there might be various possible causes, but one of these is preferred above others as being more probable.

Fallacious reasoning

Flawed reasoning in arguments is known as fallacious reasoning. Bad reasoning within arguments can be because it commits either a formal fallacy or an informal fallacy. 

Formal fallacies occur when there is a problem with the form, or structure, of the argument. The word "formal" refers to this link to the form of the argument. An argument that contains a formal fallacy will always be invalid. 

An informal fallacy is an error in reasoning that occurs due to a problem with the content, rather than mere structure, of the argument.

Traditional problems raised concerning reason

Philosophy is sometimes described as a life of reason, with normal human reason pursued in a more consistent and dedicated way than usual. Two categories of problem concerning reason have long been discussed by philosophers concerning reason, essentially being reasonings about reasoning itself as a human aim, or philosophizing about philosophizing. The first question is concerning whether we can be confident that reason can achieve knowledge of truth better than other ways of trying to achieve such knowledge. The other question is whether a life of reason, a life that aims to be guided by reason, can be expected to achieve a happy life more so than other ways of life (whether such a life of reason results in knowledge or not).

Reason versus truth, and "first principles"

Since classical times a question has remained constant in philosophical debate (which is sometimes seen as a conflict between movements called Platonism and Aristotelianism) concerning the role of reason in confirming truth. People use logic, deduction, and induction, to reach conclusions they think are true. Conclusions reached in this way are considered, according to Aristotle, more certain than sense perceptions on their own. On the other hand, if such reasoned conclusions are only built originally upon a foundation of sense perceptions, then, our most logical conclusions can never be said to be certain because they are built upon the very same fallible perceptions they seek to better.

This leads to the question of what types of first principles, or starting points of reasoning, are available for someone seeking to come to true conclusions. In Greek, "first principles" are archai, "starting points", and the faculty used to perceive them is sometimes referred to in Aristotle and Plato as nous which was close in meaning to awareness or consciousness.

Empiricism (sometimes associated with Aristotle but more correctly associated with British philosophers such as John Locke and David Hume, as well as their ancient equivalents such as Democritus) asserts that sensory impressions are the only available starting points for reasoning and attempting to attain truth. This approach always leads to the controversial conclusion that absolute knowledge is not attainable. Idealism, (associated with Plato and his school), claims that there is a "higher" reality, from which certain people can directly arrive at truth without needing to rely only upon the senses, and that this higher reality is therefore the primary source of truth. 

Philosophers such as Plato, Aristotle, Al-Farabi, Avicenna, Averroes, Maimonides, Aquinas and Hegel are sometimes said to have argued that reason must be fixed and discoverable—perhaps by dialectic, analysis, or study. In the vision of these thinkers, reason is divine or at least has divine attributes. Such an approach allowed religious philosophers such as Thomas Aquinas and Étienne Gilson to try to show that reason and revelation are compatible. According to Hegel, "...the only thought which Philosophy brings with it to the contemplation of History, is the simple conception of reason; that reason is the Sovereign of the World; that the history of the world, therefore, presents us with a rational process."

Since the 17th century rationalists, reason has often been taken to be a subjective faculty, or rather the unaided ability (pure reason) to form concepts. For Descartes, Spinoza and Leibniz, this was associated with mathematics. Kant attempted to show that pure reason could form concepts (time and space) that are the conditions of experience. Kant made his argument in opposition to Hume, who denied that reason had any role to play in experience.

Reason versus emotion or passion

After Plato and Aristotle, western literature often treated reason as being the faculty that trained the passions and appetites. Stoic philosophy by contrast considered all passions undesirable. After the critiques of reason in the early Enlightenment the appetites were rarely discussed or conflated with the passions. Some Enlightenment camps took after the Stoics to say Reason should oppose Passion rather than order it, while others like the Romantics believed that Passion displaces Reason, as in the maxim "follow your heart".

Reason has been seen as a slave, or judge, of the passions, notably in the work of David Hume, and more recently of Freud. Reasoning which claims that the object of a desire is demanded by logic alone is called rationalization.

Rousseau first proposed, in his second Discourse, that reason and political life is not natural and possibly harmful to mankind. He asked what really can be said about what is natural to mankind. What, other than reason and civil society, "best suits his constitution"? Rousseau saw "two principles prior to reason" in human nature. First we hold an intense interest in our own well-being. Secondly we object to the suffering or death of any sentient being, especially one like ourselves. These two passions lead us to desire more than we could achieve. We become dependent upon each other, and on relationships of authority and obedience. This effectively puts the human race into slavery. Rousseau says that he almost dares to assert that nature does not destine men to be healthy. According to Velkley, "Rousseau outlines certain programs of rational self-correction, most notably the political legislation of the Contrat Social and the moral education in Émile. All the same, Rousseau understands such corrections to be only ameliorations of an essentially unsatisfactory condition, that of socially and intellectually corrupted humanity."

This quandary presented by Rousseau led to Kant's new way of justifying reason as freedom to create good and evil. These therefore are not to be blamed on nature or God. In various ways, German Idealism after Kant, and major later figures such Nietzsche, Bergson, Husserl, Scheler, and Heidegger, remain preoccupied with problems coming from the metaphysical demands or urges of reason. The influence of Rousseau and these later writers is also large upon art and politics. Many writers (such as Nikos Kazantzakis) extol passion and disparage reason. In politics modern nationalism comes from Rousseau's argument that rationalist cosmopolitanism brings man ever further from his natural state.

Another view on reason and emotion was proposed in the 1994 book titled Descartes' Error by Antonio Damasio. In it, Damasio presents the "Somatic Marker Hypothesis" which states that emotions guide behavior and decision-making. Damasio argues that these somatic markers (known collectively as "gut feelings") are "intuitive signals" that direct our decision making processes in a certain way that cannot be solved with rationality alone. Damasio further argues that rationality requires emotional input in order to function.

Reason versus faith or tradition

There are many religious traditions, some of which are explicitly fideist and others of which claim varying degrees of rationalism. Secular critics sometimes accuse all religious adherents of irrationality, since they claim such adherents are guilty of ignoring, suppressing, or forbidding some kinds of reasoning concerning some subjects (such as religious dogmas, moral taboos, etc.). Though the theologies and religions such as classical monotheism typically do not claim to be irrational, there is often a perceived conflict or tension between faith and tradition on the one hand, and reason on the other, as potentially competing sources of wisdom, law and truth.

Religious adherents sometimes respond by arguing that faith and reason can be reconciled, or have different non-overlapping domains, or that critics engage in a similar kind of irrationalism:

• Reconciliation: Philosopher Alvin Plantinga argues that there is no real conflict between reason and classical theism because classical theism explains (among other things) why the universe is intelligible and why reason can successfully grasp it.
• Non-overlapping magisteria: Evolutionary biologist Stephen Jay Gould argues that there need not be conflict between reason and religious belief because they are each authoritative in their own domain (or "magisterium"). For example, perhaps reason alone is not enough to explain such big questions as the origins of the universe, the origin of life, the origin of consciousness, the foundation of morality, or the destiny of the human race. If so, reason can work on those problems over which it has authority while other sources of knowledge or opinion can have authority on the big questions.
• Tu quoque: Philosophers Alasdair MacIntyre and Charles Taylor argue that those critics of traditional religion who are adherents of secular liberalism are also sometimes guilty of ignoring, suppressing, and forbidding some kinds of reasoning about subjects. Similarly, philosophers of science such as Paul Feyaraband argue that scientists sometimes ignore or suppress evidence contrary to the dominant paradigm.
• Unification: Theologian Joseph Ratzinger, later Benedict XVI, asserted that "Christianity has understood itself as the religion of the Logos, as the religion according to reason," referring to John 1:Ἐν ἀρχῇ ἦν ὁ λόγος, usually translated as "In the beginning was the Word (Logos)." Thus, he said that the Christian faith is "open to all that is truly rational", and that the rationality of Western Enlightenment "is of Christian origin".

Some commentators have claimed that Western civilization can be almost defined by its serious testing of the limits of tension between "unaided" reason and faith in "revealed" truths—figuratively summarized as Athens and Jerusalem, respectively. Leo Strauss spoke of a "Greater West" that included all areas under the influence of the tension between Greek rationalism and Abrahamic revelation, including the Muslim lands. He was particularly influenced by the great Muslim philosopher Al-Farabi. To consider to what extent Eastern philosophy might have partaken of these important tensions, Strauss thought it best to consider whether dharma or tao may be equivalent to Nature (by which we mean physis in Greek). According to Strauss the beginning of philosophy involved the "discovery or invention of nature" and the "pre-philosophical equivalent of nature" was supplied by "such notions as 'custom' or 'ways'", which appear to be really universal in all times and places. The philosophical concept of nature or natures as a way of understanding archai (first principles of knowledge) brought about a peculiar tension between reasoning on the one hand, and tradition or faith on the other.

Although there is this special history of debate concerning reason and faith in the Islamic, Christian and Jewish traditions, the pursuit of reason is sometimes argued to be compatible with the other practice of other religions of a different nature, such as Hinduism, because they do not define their tenets in such an absolute way.

Reason in particular fields of study

Reason in political philosophy and ethics

Aristotle famously described reason (with language) as a part of human nature, which means that it is best for humans to live "politically" meaning in communities of about the size and type of a small city state (polis in Greek). For example...

It is clear, then, that a human being is more of a political [politikon = of the polis] animal [zōion] than is any bee or than any of those animals that live in herds. For nature, as we say, makes nothing in vain, and humans are the only animals who possess reasoned speech [logos]. Voice, of course, serves to indicate what is painful and pleasant; that is why it is also found in other animals, because their nature has reached the point where they can perceive what is painful and pleasant and express these to each other. But speech [logos] serves to make plain what is advantageous and harmful and so also what is just and unjust. For it is a peculiarity of humans, in contrast to the other animals, to have perception of good and bad, just and unjust, and the like; and the community in these things makes a household or city [polis]. [...] By nature, then, the drive for such a community exists in everyone, but the first to set one up is responsible for things of very great goodness. For as humans are the best of all animals when perfected, so they are the worst when divorced from law and right. The reason is that injustice is most difficult to deal with when furnished with weapons, and the weapons a human being has are meant by nature to go along with prudence and virtue, but it is only too possible to turn them to contrary uses. Consequently, if a human being lacks virtue, he is the most unholy and savage thing, and when it comes to sex and food, the worst. But justice is something political [to do with the polis], for right is the arrangement of the political community, and right is discrimination of what is just. (Aristotle's Politics 1253a 1.2. Peter Simpson's translation, with Greek terms inserted in square brackets.)

The concept of human nature being fixed in this way, implied, in other words, that we can define what type of community is always best for people. This argument has remained a central argument in all political, ethical and moral thinking since then, and has become especially controversial since firstly Rousseau's Second Discourse, and secondly, the Theory of Evolution. Already in Aristotle there was an awareness that the polis had not always existed and had needed to be invented or developed by humans themselves. The household came first, and the first villages and cities were just extensions of that, with the first cities being run as if they were still families with Kings acting like fathers.

Friendship [philia] seems to prevail [in] man and woman according to nature [kata phusin]; for people are by nature [tēi phusei] pairing [sunduastikon] more than political [politikon = of the polis], in as much as the household [oikos] is prior [proteron = earlier] and more necessary than the polis and making children is more common [koinoteron] with the animals. In the other animals, community [koinōnia] goes no further than this, but people live together [sumoikousin] not only for the sake of making children, but also for the things for life; for from the start the functions [erga] are divided, and are different [for] man and woman. Thus they supply each other, putting their own into the common [eis to koinon]. It is for these [reasons] that both utility [chrēsimon] and pleasure [hēdu] seem to be found in this kind of friendship. (Nicomachean Ethics, VIII.12.1162a. Rough literal translation with Greek terms shown in square brackets.)

Rousseau in his Second Discourse finally took the shocking step of claiming that this traditional account has things in reverse: with reason, language and rationally organized communities all having developed over a long period of time merely as a result of the fact that some habits of cooperation were found to solve certain types of problems, and that once such cooperation became more important, it forced people to develop increasingly complex cooperation—often only to defend themselves from each other. 

In other words, according to Rousseau, reason, language and rational community did not arise because of any conscious decision or plan by humans or gods, nor because of any pre-existing human nature. As a result, he claimed, living together in rationally organized communities like modern humans is a development with many negative aspects compared to the original state of man as an ape. If anything is specifically human in this theory, it is the flexibility and adaptability of humans. This view of the animal origins of distinctive human characteristics later received support from Charles Darwin's Theory of Evolution. 

The two competing theories concerning the origins of reason are relevant to political and ethical thought because, according to the Aristotelian theory, a best way of living together exists independently of historical circumstances. According to Rousseau, we should even doubt that reason, language and politics are a good thing, as opposed to being simply the best option given the particular course of events that lead to today. Rousseau's theory, that human nature is malleable rather than fixed, is often taken to imply, for example by Karl Marx, a wider range of possible ways of living together than traditionally known. 

However, while Rousseau's initial impact encouraged bloody revolutions against traditional politics, including both the French Revolution and the Russian Revolution, his own conclusions about the best forms of community seem to have been remarkably classical, in favor of city-states such as Geneva, and rural living.

Psychology

Scientific research into reasoning is carried out within the fields of psychology and cognitive science. Psychologists attempt to determine whether or not people are capable of rational thought in a number of different circumstances. 

Assessing how well someone engages in reasoning is the project of determining the extent to which the person is rational or acts rationally. It is a key research question in the psychology of reasoning. Rationality is often divided into its respective theoretical and practical counterparts.

Behavioral experiments on human reasoning

Experimental cognitive psychologists carry out research on reasoning behaviour. Such research may focus, for example, on how people perform on tests of reasoning such as intelligence or IQ tests, or on how well people's reasoning matches ideals set by logic (see, for example, the Wason test). Experiments examine how people make inferences from conditionals e.g., If A then B and how they make inferences about alternatives, e.g., A or else B. They test whether people can make valid deductions about spatial and temporal relations, e.g., A is to the left of B, or A happens after B, and about quantified assertions, e.g., All the A are B. Experiments investigate how people make inferences about factual situations, hypothetical possibilities, probabilities, and counterfactual situations.

Developmental studies of children's reasoning

Developmental psychologists investigate the development of reasoning from birth to adulthood. Piaget's theory of cognitive development was the first complete theory of reasoning development. Subsequently, several alternative theories were proposed, including the neo-Piagetian theories of cognitive development.

Neuroscience of reasoning

The biological functioning of the brain is studied by neurophysiologists and neuropsychologists. Research in this area includes research into the structure and function of normally functioning brains, and of damaged or otherwise unusual brains. In addition to carrying out research into reasoning, some psychologists, for example, clinical psychologists and psychotherapists work to alter people's reasoning habits when they are unhelpful.

Computer science

Automated reasoning

In artificial intelligence and computer science, scientists study and use automated reasoning for diverse applications including automated theorem proving the formal semantics of programming languages, and formal specification in software engineering.

Meta-reasoning

Meta-reasoning is reasoning about reasoning. In computer science, a system performs meta-reasoning when it is reasoning about its own operation. This requires a programming language capable of reflection, the ability to observe and modify its own structure and behaviour.

Evolution of reason

Dan Sperber believes that reasoning in groups is more effective and promotes their evolutionary fitness.

 

A species could benefit greatly from better abilities to reason about, predict and understand the world. French social and cognitive scientists Dan Sperber and Hugo Mercier argue that there could have been other forces driving the evolution of reason. They point out that reasoning is very difficult for humans to do effectively, and that it is hard for individuals to doubt their own beliefs (confirmation bias). Reasoning is most effective when it is done as a collective – as demonstrated by the success of projects like science. They suggest that there are not just individual, but group selection pressures at play. Any group that managed to find ways of reasoning effectively would reap benefits for all its members, increasing their fitness. This could also help explain why humans, according to Sperber, are not optimized to reason effectively alone. Their argumentative theory of reasoning claims that reason may have more to do with winning arguments than with the search for the truth.