Quantum foundations

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https://en.wikipedia.org/wiki/Quantum_foundations 

Quantum foundations is a discipline of science that seeks to understand the most counter-intuitive aspects of quantum theory, reformulate it and even propose new generalizations thereof. Contrary to other physical theories, such as general relativity, the defining axioms of quantum theory are quite ad hoc, with no obvious physical intuition. While they lead to the right experimental predictions, they do not come with a mental picture of the world where they fit.

There exist different approaches to resolve this conceptual gap:

• First, one can put quantum physics in contraposition with classical physics: by identifying scenarios, such as Bell experiments, where quantum theory radically deviates from classical predictions, one hopes to gain physical insights on the structure of quantum physics.
• Second, one can attempt to find a re-derivation of the quantum formalism in terms of operational axioms.
• Third, one can search for a full correspondence between the mathematical elements of the quantum framework and physical phenomena: any such correspondence is called an interpretation.
• Fourth, one can renounce quantum theory altogether and propose a different model of the world.

Research in quantum foundations is structured along these roads.

Non-classical features of quantum theory

Quantum nonlocality

Main article: Quantum nonlocality

Two or more separate parties conducting measurements over a quantum state can observe correlations which cannot be explained with any local hidden variable theory. Whether this should be regarded as proving that the physical world itself is "nonlocal" is a topic of debate, but the terminology of "quantum nonlocality" is commonplace. Nonlocality research efforts in quantum foundations focus on determining the exact limits that classical or quantum physics enforces on the correlations observed in a Bell experiment or more complex causal scenarios. This research program has so far provided a generalization of Bell's theorem that allows falsifying all classical theories with a superluminal, yet finite, hidden influence.

Quantum contextuality

Main article: Quantum contextuality

Nonlocality can be understood as an instance of quantum contextuality. A situation is contextual when the value of an observable depends on the context in which it is measured (namely, on which other observables are being measured as well). The original definition of measurement contextuality can be extended to state preparations and even general physical transformations.

Epistemic models for the quantum wave-function

A physical property is epistemic when it represents our knowledge or beliefs on the value of a second, more fundamental feature. The probability of an event to occur is an example of an epistemic property. In contrast, a non-epistemic or ontic variable captures the notion of a “real” property of the system under consideration.

There is an on-going debate on whether the wave-function represents the epistemic state of a yet to be discovered ontic variable or, on the contrary, it is a fundamental entity. Under some physical assumptions, the Pusey–Barrett–Rudolph (PBR) theorem demonstrates the inconsistency of quantum states as epistemic states, in the sense above. Note that, in QBism and Copenhagen-type views, quantum states are still regarded as epistemic, not with respect to some ontic variable, but to one's expectations about future experimental outcomes. The PBR theorem does not exclude such epistemic views on quantum states.

Axiomatic reconstructions

Some of the counter-intuitive aspects of quantum theory, as well as the difficulty to extend it, follow from the fact that its defining axioms lack a physical motivation. An active area of research in quantum foundations is therefore to find alternative formulations of quantum theory which rely on physically compelling principles. Those efforts come in two flavors, depending on the desired level of description of the theory: the so-called Generalized Probabilistic Theories approach and the Black boxes approach.

The framework of Generalized Probabilistic Theories

Main article: Generalized probabilistic theories

Generalized Probabilistic Theories (GPTs) are a general framework to describe the operational features of arbitrary physical theories. Essentially, they provide a statistical description of any experiment combining state preparations, transformations and measurements. The framework of GPTs can accommodate classical and quantum physics, as well as hypothetical non-quantum physical theories which nonetheless possess quantum theory's most remarkable features, such as entanglement or teleportation. Notably, a small set of physically motivated axioms is enough to single out the GPT representation of quantum theory.

L. Hardy introduced the concept of GPT in 2001, in an attempt to re-derive quantum theory from basic physical principles. Although Hardy's work was very influential (see the follow-ups below), one of his axioms was regarded as unsatisfactory: it stipulated that, of all the physical theories compatible with the rest of the axioms, one should choose the simplest one. The work of Dakic and Brukner eliminated this “axiom of simplicity” and provided a reconstruction of quantum theory based on three physical principles. This was followed by the more rigorous reconstruction of Masanes and Müller.

Axioms common to these three reconstructions are:

• The subspace axiom: systems which can store the same amount of information are physically equivalent.
• Local tomography: to characterize the state of a composite system it is enough to conduct measurements at each part.
• Reversibility: for any two extremal states [i.e., states which are not statistical mixtures of other states], there exists a reversible physical transformation that maps one into the other.

An alternative GPT reconstruction proposed by Chiribella et al. around the same time is also based on the

• Purification axiom: for any state ${\displaystyle S_{A}}$ of a physical system A there exists a bipartite physical system ${\displaystyle A-B}$ and an extremal state (or purification) ${\displaystyle T_{AB}}$ such that ${\displaystyle S_{A}}$ is the restriction of ${\displaystyle T_{AB}}$ to system ${\displaystyle A}$. In addition, any two such purifications ${\displaystyle T_{AB},T_{AB}^{\prime }}$ of ${\displaystyle S_{A}}$ can be mapped into one another via a reversible physical transformation on system ${\displaystyle B}$.

The use of purification to characterize quantum theory has been criticized on the grounds that it also applies in the Spekkens toy model.

To the success of the GPT approach, it can be countered that all such works just recover finite dimensional quantum theory. In addition, none of the previous axioms can be experimentally falsified unless the measurement apparatuses are assumed to be tomographically complete.

Categorical Quantum Mechanics or Process Theories

Main article: Categorical quantum mechanics

Categorical Quantum Mechanics (CQM) or Process Theories are a general framework to describe physical theories, with an emphasis on processes and their compositions. It was pioneered by Samson Abramsky and Bob Coecke. Besides its influence in quantum foundations, most notably the use of a diagrammatic formalism, CQM also plays an important role in quantum technologies, most notably in the form of ZX-calculus. It also has been used to model theories outside of physics, for example the DisCoCat compositional natural language meaning model.

The framework of black boxes

Main article: Quantum nonlocality

In the black box or device-independent framework, an experiment is regarded as a black box where the experimentalist introduces an input (the type of experiment) and obtains an output (the outcome of the experiment). Experiments conducted by two or more parties in separate labs are hence described by their statistical correlations alone.

From Bell's theorem, we know that classical and quantum physics predict different sets of allowed correlations. It is expected, therefore, that far-from-quantum physical theories should predict correlations beyond the quantum set. In fact, there exist instances of theoretical non-quantum correlations which, a priori, do not seem physically implausible. The aim of device-independent reconstructions is to show that all such supra-quantum examples are precluded by a reasonable physical principle.

The physical principles proposed so far include no-signalling, Non-Trivial Communication Complexity, No-Advantage for Nonlocal computation, Information Causality, Macroscopic Locality, and Local Orthogonality. All these principles limit the set of possible correlations in non-trivial ways. Moreover, they are all device-independent: this means that they can be falsified under the assumption that we can decide if two or more events are space-like separated. The drawback of the device-independent approach is that, even when taken together, all the afore-mentioned physical principles do not suffice to single out the set of quantum correlations. In other words: all such reconstructions are partial.

Interpretations of quantum theory

Main article: Interpretations of quantum mechanics

An interpretation of quantum theory is a correspondence between the elements of its mathematical formalism and physical phenomena. For instance, in the pilot wave theory, the quantum wave function is interpreted as a field that guides the particle trajectory and evolves with it via a system of coupled differential equations. Most interpretations of quantum theory stem from the desire to solve the quantum measurement problem.

Extensions of quantum theory

In an attempt to reconcile quantum and classical physics, or to identify non-classical models with a dynamical causal structure, some modifications of quantum theory have been proposed.

Collapse models

Collapse models posit the existence of natural processes which periodically localize the wave-function. Such theories provide an explanation to the nonexistence of superpositions of macroscopic objects, at the cost of abandoning unitarity and exact energy conservation.

Quantum Measure Theory

In Sorkin's quantum measure theory (QMT), physical systems are not modeled via unitary rays and Hermitian operators, but through a single matrix-like object, the decoherence functional. The entries of the decoherence functional determine the feasibility to experimentally discriminate between two or more different sets of classical histories, as well as the probabilities of each experimental outcome. In some models of QMT the decoherence functional is further constrained to be positive semidefinite (strong positivity). Even under the assumption of strong positivity, there exist models of QMT which generate stronger-than-quantum Bell correlations.

Acausal quantum processes

The formalism of process matrices starts from the observation that, given the structure of quantum states, the set of feasible quantum operations follows from positivity considerations. Namely, for any linear map from states to probabilities one can find a physical system where this map corresponds to a physical measurement. Likewise, any linear transformation that maps composite states to states corresponds to a valid operation in some physical system. In view of this trend, it is reasonable to postulate that any high-order map from quantum instruments (namely, measurement processes) to probabilities should also be physically realizable. Any such map is termed a process matrix. As shown by Oreshkov et al., some process matrices describe situations where the notion of global causality breaks.

The starting point of this claim is the following mental experiment: two parties, Alice and Bob, enter a building and end up in separate rooms. The rooms have ingoing and outgoing channels from which a quantum system periodically enters and leaves the room. While those systems are in the lab, Alice and Bob are able to interact with them in any way; in particular, they can measure some of their properties.

Since Alice and Bob's interactions can be modeled by quantum instruments, the statistics they observe when they apply one instrument or another are given by a process matrix. As it turns out, there exist process matrices which would guarantee that the measurement statistics collected by Alice and Bob is incompatible with Alice interacting with her system at the same time, before or after Bob, or any convex combination of these three situations. Such processes are called acausal.

Altruism (biology)

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https://en.wikipedia.org/wiki/Altruism_(biology) 

In biology, altruism refers to behaviour by an individual that increases the fitness of another individual while decreasing the fitness of the agent. Altruism in this sense is different from the philosophical concept of altruism, in which an action would only be called "altruistic" if it was done with the conscious intention of helping another. In the behavioural sense, there is no such requirement. As such, it is not evaluated in moral terms—it is the consequences of an action for reproductive fitness that determine whether the action is considered altruistic, not the intentions, if any, with which the action is performed.

The term altruism was coined by the French philosopher Auguste Comte in French, as altruisme, for an antonym of egoism. He derived it from the Italian altrui, which in turn was derived from Latin alteri, meaning "other people" or "somebody else".

Altruistic behaviours appear most obviously in kin relationships, such as in parenting, but may also be evident among wider social groups, such as in social insects. They allow an individual to increase the success of its genes by helping relatives that share those genes. Obligate altruism is the permanent loss of direct fitness (with potential for indirect fitness gain). For example, honey bee workers may forage for the colony. Facultative altruism is temporary loss of direct fitness (with potential for indirect fitness gain followed by personal reproduction). For example, a Florida scrub jay may help at the nest, then gain parental territory.

Overview

In ethology (the study of behavior), and more generally in the study of social evolution, on occasion, some animals do behave in ways that reduce their individual fitness but increase the fitness of other individuals in the population; this is a functional definition of altruism. Research in evolutionary theory has been applied to social behaviour, including altruism. Cases of animals helping individuals to whom they are closely related can be explained by kin selection, and are not considered true altruism. Beyond the physical exertions that in some species mothers and in some species fathers undertake to protect their young, extreme examples of sacrifice may occur. One example is matriphagy (the consumption of the mother by her offspring) in the spider Stegodyphus; another example is a male spider allowing a female fertilized by him to eat him. Hamilton's rule describes the benefit of such altruism in terms of Wright's coefficient of relationship to the beneficiary and the benefit granted to the beneficiary minus the cost to the sacrificer. Should this sum be greater than zero a fitness gain will result from the sacrifice.

When apparent altruism is not between kin, it may be based on reciprocity. A monkey will present its back to another monkey, who will pick out parasites; after a time the roles will be reversed. Such reciprocity will pay off, in evolutionary terms, as long as the costs of helping are less than the benefits of being helped and as long as animals will not gain in the long run by "cheating"—that is to say, by receiving favours without returning them. This is elaborated on in evolutionary game theory and specifically the prisoner's dilemma as social theory.

Implications in evolutionary theory

Cooperative hunting by wolves allows them to tackle much larger and more nutritious prey than any individual wolf could handle. However, such cooperation could, potentially, be exploited by selfish individuals who do not expose themselves to the dangers of the hunt, but nevertheless share in the spoils.

The existence of altruism in nature is at first sight puzzling, because altruistic behaviour reduces the likelihood that an individual will reproduce. The idea that group selection might explain the evolution of altruism was first broached by Darwin himself in The Descent of Man, and Selection in Relation to Sex, (1871). The concept of group selection has had a chequered and controversial history in evolutionary biology but the uncritical 'good of the species' tradition came to an abrupt halt in the 1960s, due largely to the work of George C. Williams, and John Maynard Smith as well as Richard Dawkins. These evolutionary theorists pointed out that natural selection acts on the individual, and that it is the individual's fitness (number of offspring and grand-offspring produced compared to the rest of the population) that drives evolution. A group advantage (e.g. hunting in a pack) that is disadvantageous to the individual (who might be harmed during the hunt, when it could avoid injury by hanging back from the pack but still share in the spoils) cannot evolve, because the selfish individual will leave, on average, more offspring than those who join the pack and suffer injuries as a result. If the selfishness is hereditary, this will ultimately result in the population consisting entirely of selfish individuals. However, in the 1960s and 1970s an alternative to the "group selection" theory emerged. This was the kin selection theory, due originally to W. D. Hamilton. Kin selection is an instance of inclusive fitness, which is based on the notion that an individual shares only half its genes with each offspring, but also with each full sibling. From an evolutionary genetic point of view it is therefore as advantageous to help with the upbringing of full sibs as it is to produce and raise one's own offspring. The two activities are evolutionarily entirely equivalent. Co-operative breeding (i.e. helping one's parents raise sibs—provided they are full sibs) could thus evolve without the need for group-level selection. This quickly gained prominence among biologists interested in the evolution of social behaviour.

Olive baboons grooming

In 1971 Robert Trivers introduced his reciprocal altruism theory to explain the evolution of helping at the nest of an unrelated breeding pair of birds. He argued that an individual might act as a helper if there was a high probabilistic expectation of being helped by the recipients at some later date. If, however, the recipients did not reciprocate when it was possible to do so, the altruistic interaction with these recipients would be permanently terminated. But if the recipients did not cheat then the reciprocal altruism would continue indefinitely to both parties' advantage. This model was considered by many (e.g. West-Eberhard and Dawkins) to be evolutionarily unstable because it is prone to invasion by cheats for the same reason that cooperative hunting can be invaded and replaced by cheats. However, Trivers did make reference to the Prisoner's Dilemma Game which, 10 years later, would restore interest in Trivers' reciprocal altruism theory, but under the title of "tit-for-tat".

In its original form the Prisoner's Dilemma Game (PDG) described two awaiting trial prisoners, A and B, each faced with the choice of betraying the other or remaining silent. The "game" has four possible outcomes: (a) they both betray each other, and are both sentenced to two years in prison; (b) A betrays B, which sets A free and B is sentenced to four years in prison; (c) B betrays A, with the same result as (b) except that it is B who is set free and the other spends four years in jail; (d) both remain silent, resulting in a six-month sentence each. Clearly (d) ("cooperation") is the best mutual strategy, but from the point of view of the individual betrayal is unbeatable (resulting in being set free, or getting only a two-year sentence). Remaining silent results in a four-year or six-month sentence. This is exemplified by a further example of the PDG: two strangers attend a restaurant together and decide to split the bill. The mutually best ploy would be for both parties to order the cheapest items on the menu (mutual cooperation). But if one member of the party exploits the situation by ordering the most expensive items, then it is best for the other member to do likewise. In fact, if the fellow diner's personality is completely unknown, and the two diners are unlikely ever to meet again, it is always in one's own best interests to eat as expensively as possible. Situations in nature that are subject to the same dynamics (rewards and penalties) as the PDG define cooperative behaviour: it is never in the individual's fitness interests to cooperate, even though mutual cooperation rewards the two contestants (together) more highly than any other strategy. Cooperation cannot evolve under these circumstances.

However, in 1981 Axelrod and Hamilton noted that if the same contestants in the PDG meet repeatedly (the so-called Iterated Prisoner's Dilemma game, IPD) then tit-for-tat (foreshadowed by Robert Triver's reciprocal altruism theory) is a robust strategy which promotes altruism. In "tit-for-tat" both players' opening moves are cooperation. Thereafter each contestant repeats the other player's last move, resulting in a seemingly endless sequence of mutually cooperative moves. However, mistakes severely undermine tit-for-tat's effectiveness, giving rise to prolonged sequences of betrayal, which can only be rectified by another mistake. Since these initial discoveries, all the other possible IPD game strategies have been identified (16 possibilities in all, including, for instance, "generous tit-for-tat", which behaves like "tit-for-tat", except that it cooperates with a small probability when the opponent's last move was "betray"), but all can be outperformed by at least one of the other strategies, should one of the players switch to such a strategy. The result is that none is evolutionarily stable, and any prolonged series of the iterated prisoner's dilemma game, in which alternative strategies arise at random, gives rise to a chaotic sequence of strategy changes that never ends.

The handicap principle

 

A male peacock with its beautiful but clumsy, aerodynamically unsound tail—a handicap, comparable to a race horse's handicap.

 

The best horses in a handicap race carry the largest weights, so the size of the handicap is a measure of the animal's quality

In the light of the Iterated Prisoner's Dilemma Game failing to provide a full answer to the evolution of cooperation or altruism, several alternative explanations have been proposed.

There are striking parallels between altruistic acts and exaggerated sexual ornaments displayed by some animals, particularly certain bird species, such as, amongst others, the peacock. Both are costly in fitness terms, and both are generally conspicuous to other members of the population or species. This led Amotz Zahavi to suggest that both might be fitness signals rendered evolutionarily stable by his handicap principle. If a signal is to remain reliable, and generally resistant to falsification, the signal has to be evolutionarily costly. Thus, if a (low fitness) liar were to use the highly costly signal, which seriously eroded its real fitness, it would find it difficult to maintain a semblance or normality. Zahavi borrowed the term "handicap principle" from sports handicapping systems. These systems are aimed at reducing disparities in performance, thereby making the outcome of contests less predictable. In a horse handicap race, provenly faster horses are given heavier weights to carry under their saddles than inherently slower horses. Similarly, in amateur golf, better golfers have fewer strokes subtracted from their raw scores than the less talented players. The handicap therefore correlates with unhandicapped performance, making it possible, if one knows nothing about the horses, to predict which unhandicapped horse would win an open race. It would be the one handicapped with the greatest weight in the saddle. The handicaps in nature are highly visible, and therefore a peahen, for instance, would be able to deduce the health of a potential mate by comparing its handicap (the size of the peacock's tail) with those of the other males. The loss of the male's fitness caused by the handicap is offset by its increased access to females, which is as much of a fitness concern as is its health. An altruistic act is, by definition, similarly costly. It would therefore also signal fitness, and is probably as attractive to females as a physical handicap. If this is the case altruism is evolutionarily stabilized by sexual selection.

African pygmy kingfisher, showing details of appearance and colouration that are shared by all African pygmy kingfishers to a high degree of fidelity.

There is an alternate strategy for identifying fit mates which does not rely on one gender having exaggerated sexual ornaments or other handicaps, but is generally applicable to most, if not all sexual creatures. It derives from the concept that the change in appearance and functionality caused by a non-silent mutation will generally stand out in a population. This is because that altered appearance and functionality will be unusual, peculiar, and different from the norm within that population. The norm against which these unusual features are judged is made up of fit attributes that have attained their plurality through natural selection, while less adaptive attributes will be in the minority or frankly rare. Since the overwhelming majority of mutant features are maladaptive, and it is impossible to predict evolution's future direction, sexual creatures would be expected to prefer mates with the fewest unusual or minority features. This will have the effect of a sexual population rapidly shedding peripheral phenotypic features and canalizing the entire outward appearance and behaviour so that all the members of that population will begin to look remarkably similar in every detail, as illustrated in the accompanying photograph of the African pygmy kingfisher, Ispidina picta. Once a population has become as homogeneous in appearance as is typical of most species, its entire repertoire of behaviours will also be rendered evolutionarily stable, including any altruistic, cooperative and social characteristics. Thus, in the example of the selfish individual who hangs back from the rest of the hunting pack, but who nevertheless joins in the spoils, that individual will be recognized as being different from the norm, and will therefore find it difficult to attract a mate. Its genes will therefore have only a very small probability of being passed on to the next generation, thus evolutionarily stabilizing cooperation and social interactions at whatever level of complexity is the norm in that population.

Reciprocity mechanisms

Altruism in animals describes a range of behaviors performed by animals that may be to their own disadvantage but which benefit others. The costs and benefits are measured in terms of reproductive fitness, or expected number of offspring. So by behaving altruistically, an organism reduces the number of offspring it is likely to produce itself, but boosts the likelihood that other organisms are to produce offspring. There are other forms of altruism in nature other than risk-taking behavior, such as reciprocal altruism. This biological notion of altruism is not identical to the everyday human concept. For humans, an action would only be called 'altruistic' if it was done with the conscious intention of helping another. Yet in the biological sense there is no such requirement. Instead, until we can communicate directly with other species, an accurate theory to describe altruistic acts between species is Biological Market Theory. Humans and other animals exchange benefits in several ways, known technically as reciprocity mechanism. No matter what the mechanism, the common thread is that benefits find their way back to the original giver.

Symmetry-based

Also known as the "buddy-system", mutual affection between two parties prompts similar behavior in both directions without need to track of daily give-and-take, so long as the overall relationship remains satisfactory. This is one of the most common mechanism of reciprocity in nature, this kind is present in humans, primates, and many other mammals.

Attitudinal

Also known as, "If you're nice, I'll be nice too." This mechanism of reciprocity is similar to the heuristic of the golden rule, "Treat others how you would like to be treated." Parties mirror one another's attitudes, exchanging favors on the spot. Instant attitudinal reciprocity occurs among monkeys, and people often rely on it with strangers and acquaintances.

Calculated

Also known as, "what have you done for me lately?" Individuals keep track of the benefits they exchange with particular partners, which helps them decide to whom to return favors. This mechanism is typical of chimpanzees and very common among human relationships. Yet some opposing experimental research suggests that calculated or contingent reciprocity does not spontaneously arise in laboratory experimental settings, despite patterns of behavior.

Biological market theory

Biological market theory is an extension of the idea of reciprocal altruism, as a mechanism to explain altruistic acts between unrelated individuals in a more flexible system of exchanging commodities. The term 'biological market' was first used by Ronald Noe and Hammerstein in 1994 to refer to all the interactions between organisms in which different organisms function as 'traders' that exchange goods and services such as food and water, grooming, warning calls, shelter, etc. Biological market theory consists of five formal characteristics which present a basis for altruism.

1. Commodities are exchanged between individuals that differ in the degree of control over those commodities.
2. Trading partners are chosen from a number of potential partners.
3. There is competition among the members of the chosen class to be the most attractive partner. This competition by 'outbidding' causes an increase in the value of the commodity offered.
4. Supply and demand determine the bartering value of commodities exchanged.
5. Commodities on offer can be advertised. As in commercial advertisements there is a potential for false information.

Two bluestreak cleaner wrasses cleaning a potato cod

The applicability of biological market theory with its emphasis on partner choice is evident in the interactions between the cleaner wrasse and its "client" reef fish. Cleaners have small territories, which the majority of reef fish species actively visit to invite inspection of their surface, gills, and mouth. Clients benefit from the removal of parasites while cleaners benefit from the access to a food source. Some particularly choosy client species have large home ranges that cover several cleaning stations, whereas other clients have small ranges and have access to one cleaning station only (resident clients). Field observations, field manipulations, and laboratory experiments revealed that whether or not a client has choice options influences several aspects of both cleaner and client behaviour. Cleaners give choosy clients priority of access. Choosy clients switch partners if cheated by a cleaner by taking a bite of out of the cleaner, whereas resident clients punish cheats. Cleaners and resident clients, but not choosy clients, build up relationships before normal cleaning interactions take place. Cleaners are particularly cooperative if choosy clients are bystanders of an interaction but less so when resident clients are bystanders.

Researchers tested whether wild white-handed gibbon males from Khao Yai National Park, Thailand, increased their grooming activity when the female partner was fertile. Adult females and males of our study population are codominant (in terms of aggression), they live in pairs or small multi male groups and mate promiscuously. They found that males groomed females more than vice versa and more grooming was exchanged when females were cycling than during pregnancy or lactation. The number of copulations/day was elevated when females were cycling, and females copulated more frequently with males on days when they received more grooming. When males increased their grooming efforts, females also increased their grooming of males, perhaps to equalize give and take. Although grooming might be reciprocated because of intrinsic benefits of receiving grooming, males also interchange grooming as a commodity for sexual opportunities during a female's fertile period.

Examples in vertebrates

Mammals

• Wolves and wild dogs bring meat back to members of the pack not present at the kill. Though in harsh conditions, the breeding pair of wolves take the greatest share to continue to produce pups.
• Mongooses support elderly, sick, or injured animals.
• Meerkats often have one standing guard to warn while the rest feed in case of predator attack.
• Raccoons inform conspecifics about feeding grounds by droppings left on commonly shared latrines. A similar information system has been observed to be used by common ravens.
• Male baboons threaten predators and cover the rear as the troop retreats.
• Gibbons and chimpanzees with food will, in response to a gesture, share their food with others of the group. Chimpanzees will help humans and conspecifics without any reward in return.
• Bonobos have been observed aiding injured or disabled bonobos.
• Vampire bats commonly regurgitate blood to share with unlucky or sick roost mates that have been unable to find a meal, often forming a buddy system.
• Vervet monkeys give alarm calls to warn fellow monkeys of the presence of predators, even though in doing so they attract attention to themselves, increasing their personal chance of being attacked.
• Lemurs of all ages and of both sexes will take care of infants unrelated to them.
• Dolphins support sick or injured members of their pod, swimming under them for hours at a time and pushing them to the surface so they can breathe.
• Walruses have been seen adopting orphans who lost their parents to predators.
• African buffalo will rescue a member of the herd captured by predators. (See Battle at Kruger.)
• Humpback whales have been observed protecting other species from killer whales.
• Male Przewalski's horses have been observed engaging in intervention behaviour when their group members were threatened. They did not distinguish between kin and non-kin members. It has been theorized that they may do this to promote group cohesion and reduce social disruption within the group.

Birds

• In numerous bird species, a breeding pair receives support in raising its young from other "helper" birds, including help with the feeding of its fledglings. Some will even go as far as protecting an unrelated bird's young from predators.

Fish

• Harpagifer bispinis, a species of fish, live in social groups in the harsh environment of the Antarctic Peninsula. If the parent guarding the nest of eggs is removed, a usually male replacement unrelated to the parents guards the nest from predators and prevents fungal growth that would kill off the brood. There is no clear benefit to the male so the act may be considered altruistic.

Examples in invertebrates

• Some termites, such as Globitermes sulphureus and ants, such as Camponotus saundersi release a sticky secretion by fatally rupturing a specialized gland. This autothysis altruistically defends the colony at the expense of the individual insect. This can be attributed to the fact that ants share their genes with the entire colony, and so this behaviour is evolutionarily beneficial (not necessarily for the individual ant but for the continuation of its genetic make-up).
• Synalpheus regalis is a species of eusocial marine snapping shrimp that lives in sponges in coral reefs. They live in colonies of about 300 individuals with one reproductive female. Other colony members defend the colony against intruders, forage, and care for the young. Eusociality in this system entails an adaptive division of labor which results in enhanced reproductive output of the breeders and inclusive fitness benefits for the nonbreeding helpers. S. regalis are exceptionally tolerant of conspecifics within their colonies due to close genetic relatedness among nestmates. Allozyme data reveals that relatedness within colonies is high, which is an indication that colonies in this species represent close kin groups. The existence of such groups is an important prerequisite of explanations of social evolution based on kin selection.

Examples in protists

An example of altruism is found in the cellular slime moulds, such as Dictyostelium mucoroides. These protists live as individual amoebae until starved, at which point they aggregate and form a multicellular fruiting body in which some cells sacrifice themselves to promote the survival of other cells in the fruiting body.

Examples in plants

When it comes to altruism in kin/non-kin recognition, few studies have focused on this trait in crops. Despite most crops growing in monocultures, there is evidence that they are able to recognize kin and other cultivars. For example, cultivated soybean plants were able to recognize a distant ancestor and unrelated neighbors. In that experiment, plants were grown in combinations of relation to each other (same cultivar or different cultivar) in pots and their biomass of stems, leaves, and roots were measured to see how the plants responded growing next to kin or non-kin. Crops, unlike wild plants, are highly cultivated. The evolution of traits such as altruism can thus be bred into them through the selection of the trait. In agriculture, the importance of yield is stressed, therefore breeding crop cultivars to favor altruism can decrease competitiveness and increase yield. It has been shown that using mass selection early in the breeding process selects against altruism in an individual, but using mixed individual and group selection favors altruism.

Christian vegetarianism

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Christian_vegetarianism 

Christian vegetarianism is the practice of keeping to a vegetarian lifestyle for reasons connected to or derived from the Christian faith. The three primary reasons are spiritual, nutritional, and ethical. The ethical reasons may include a concern for God's creation, a concern for animal rights and welfare, or both. Likewise, Christian veganism is not using any animal products for reasons connected to or derived from the Christian faith.

Pescatarianism was widespread in the early Church, among both the clergy and laity.

Among the early Judeo-Christian Gnostics the Ebionites held that John the Baptist, James the Just and Jesus were vegetarians.

Some religious orders of various Christian Churches practice pescatarianism, including the Benedictines, Franciscans, Trappists, Carthusians and Cistercians. Various Church leaders have recommended vegetarianism, including John Wesley (founder of the Methodist Church), William and Catherine Booth (founders of The Salvation Army), William Cowherd from the Bible Christian Church and Ellen G. White from the Seventh-day Adventists. Cowherd, who founded the Bible Christian Church in 1809, helped to establish the world's first Vegetarian Society in 1847.

Organizations such as the Christian Vegetarian Association (CVA) work to promote the concept.

Additionally, many Christians may choose to practice vegetarianism or veganism as their Lenten sacrifice during Lent periods.

Biblical support

Christian vegetarianism has not been a common dietary choice throughout Church history. Some have argued, however, that "there is a long-standing tradition of vegetarianism in Christian history." The two most prominent forms are a spirituality-based vegetarianism (where vegetarianism is adopted as an ascetic practice, or as a way of opposing the sin of gluttony, in the hope it will draw the person to God) and an ethically-based vegetarianism (where it is adopted for ethical reasons; for example, those to do with the treatment of non-human animals). Christian ethical vegetarianism (or veganism) usually carries with it a commitment to the normative claim that (at least some) Christians should be vegetarians. For this reason, Christian ethical vegetarians often give a scriptural justification for their position. While there are biblical passages which provide support for ethical vegetarianism, there are also passages which seem to imply that eating animals is morally permissible.

Old Testament

See also: Jewish vegetarianism

One of the most important passages for Christian vegetarians is the creation narrative in the Book of Genesis. After creating humans, God addresses them in chapter 1, verses 1:29–30 as follows:

God said, "See, I have given you every plant yielding seed that is upon the face of all the earth, and every tree with seed in its fruit; you shall have them for food. And to every beast of the earth, and to every bird of the air, and to everything that creeps on the earth, everything that has the breath of life, I have given every green plant for food". And it was so.

In this passage, God prescribes a plant-based diet not just for humans, but for all land-based non-human animals. Christian vegetarians and vegans point out that it was this creation—where all creatures ate plants—that God then declared "very good" in verse 31. Moreover, that God's initial creation was a vegan creation suggests that this is how God intended all his creatures to live. This idea—that God intended for all his creatures to eat plants—is sometimes further supported by noting that the vision of the Peaceable Kingdom found in the Book of Isaiah 11:6–9 suggests that, one day, God will restore the creation to such a state of universal vegetarianism:

The wolf shall live with the lamb, the leopard shall lie down with the kid, the calf and the lion and the fatling together, and a little child shall lead them. The cow and the bear shall graze, their young shall lie down together; and the lion shall eat straw like the ox. The nursing child shall play over the hole of the asp, and the weaned child shall put its hand on the adder’s den. They will not hurt or destroy on all my holy mountain; for the earth will be full of the knowledge of the Lord as the waters cover the sea.

Some Christian vegetarians have suggested that this eschatological view provides reasons to adopt a vegetarian or vegan diet here and now. Moreover, the point has often been made that the dominion which humans are given over the non-human animals in Genesis 1:26–28 must be understood in light of Genesis 1:29–30 which decrees a plant-based diet for all creatures. Genesis 1:26–28 has, it is acknowledged by Christian vegetarians, often been used to justify the eating of animals. But this is a mistake, they suggest. Once it is recognized that humans are given dominion over creation, and that in the very next verse humans are prescribed a plant-based diet, it will become apparent that dominion should be understood in terms of stewardship or servant-hood: humans are called to rule creation in the sense of caring for it and seeking its flourishing, just as a good Sovereign would seek the flourishing of his or her realm. In a survey of the scholarly literature on the relevant Hebrew terms, Carol J. Adams lists governing, ruling, shepherding, caring-for, nurturing, and leading about as potential ways of understanding dominion, and notes that the common characteristic of these concepts "is their benignity".

The opening chapters of Genesis are, of course, only the beginning of the biblical story. And just as there are passages which can be cited in support of a Christian vegetarianism or veganism, so there are passages which suggest that eating animals is morally permissible. The most problematic passages for Christian vegetarians are those which include an explicit permission to eat animals. Genesis 9:3–4 is the first such example. In this verse, God tells Noah and his family that animals will now be their food, although they are not to eat animal flesh which contains blood. This new situation – that of humans eating animals – is then taken largely for granted in much of the biblical narrative. Leviticus 11 records God giving the Israelites rules about what types of meat may be eaten, which implies that certain meats were acceptable. During the Exodus out of Egypt, God commanded that all of the Israelites to slaughter a lamb and eat it, and instituted the Passover as a lasting tradition to remember God's saving them.^{[Exodus 12:24]}

Some Jewish and Christian vegetarians have attempted to minimize the importance of these passages. It has been suggested, for example, that God's permitting Noah and his family to eat meat was only ever intended as a temporary permission, and was given because all the plants had been destroyed as a result of the flood. Others interpret the permission given to Noah and his family in Genesis 9:3–4, not as a free pass to kill animals for food because "no matter what you do you can never remove all the blood from the flesh of a slaughtered animal", but as an invitation to scavenge for and eat dead animals if any are found. These approaches are put under pressure, however, with the sheer number of passages which appear to presuppose the legitimacy of eating animals, and the normalcy with which meat eating is treated.

Another approach to these texts is to suggest that God reluctantly permitted the eating of animals due to human sinfulness. In other words, God permitted humans to eat non-human animals as a concession to the Fallen state of humanity. Richard Young raises the possibility that both the introduction of animals into the human diet, and the use of animals in religious sacrifices, were concessions to a Fallen humanity that were used to deal with humanity where it was at. This approach allows the Christian vegetarian or vegan to take the entire biblical witness seriously, while also holding that God's preference is for a peace and shalom throughout creation.

Other passages of relevance to the practice of vegetarianism include Numbers 11, where the Israelites tired of manna, a food of which "The Rabbis of the Talmud held that […] had whatever taste and flavor the eater desired at the time of eating" and which probably was not an animal product. Manna was given to the Israelites by God, but they complained about it and wanted meat instead.^{[Numbers 11:4–10]} They were condemned for this, although God relented and gave them meat, which then made them ill.^{[Numbers 11:32–34]} Because of their lust, the place where the incident happened became known as Kibroth Hattaavah.

A donkey temporarily given the ability to speak showed Balaam more than signs of sentience.^{[Numbers 22:21–33]}

Some people believe that the Book of Daniel also specifically promotes veganism as empowering. Daniel specifically refuses the king's "meat" (paṯbaḡ, Strong's #5698) and instead requests vegetables (zērōʿîm, Strong's #2235).^{[Daniel 1:8–16]} However, current common theology argues that in this instance Daniel, Hananiah, Mishael and Azariah are rejecting food that is considered to be unholy by their faith (eating food that had been sacrificed to pagan gods), and not meat per se, despite that "at the end of ten days their countenances appeared fairer and fatter in flesh than all the children which did eat the portion of the king's meat".^{[Daniel 1:15]}

Philo says that the Essenes, "being more scrupulous than any in the worship of God […] do not sacrifice animals […], but hold it right to dedicate their own hearts as a worthy offering". They maintained that the sacrifices "polluted" the Temple.

The Christian Vegetarian Association of the UK claims that the word "meat" is not used in any one instance in the authorized version of either the Old or New Testament as relating only to animal food (e.g. "flesh"). The CVA states that when the first English translations of the Bible were created, the word for "meat" meant food in general. When any particular kind of food was designated, it was referred to as meal, flour or grain.

According to the CVS, examples of New Testament words that were translated as "meat" include: broma ("that which is eaten"/usage: 16 times ); brosimos ("eatable"/usage: 1 time); brosis ("act of eating; that which is eaten, food; food of the souls/usage: 7 times); prosphagion ("anything eaten with bread; spoken of fish boiled or broiled"/usage: 1 time); sitometron ("a measured portion of grain or food"/usage: 1 time); trapeza ("a table on which food is placed, an eating place"/usage: 1 time); trophe ("food, nourishment"/usage: 13 times); phago ("to eat, to take food, eat a meal, devour, consume"/usage: 3 times).

New Testament

The case for Christian vegetarianism

Christian vegetarians and vegans often appeal to the ethic that Jesus embodied, and called people to emulate, in making their case for a distinctively Christian vegetarianism. To begin with, Jesus inaugurated the Kingdom of God, but his Kingdom didn't involve the exercise of power as humans tend to think of it. As Andrew Linzey argues, Christ's power is "the power to serve". Human beings are called to have the same mind that was found in Jesus Christ, i.e., the mind to exercise power in service.^{[Philippians 2:5–9]} And by considering Jesus's life, it is possible to get an idea of what that service means. Sarah Withrow King writes that Jesus "loved the unlovable. In first-century Palestine, the unlovable were women, children, sick people, poor people, Roman soldiers, zealots, lepers, the blind, the outcast", and so on. But today, the unlovable should include those non-human animals who are farmed for food in systems which preclude their flourishing and result in their (often painful) deaths.

Christian vegetarians also stress the importance Jesus laid on peace and inclusion. These and other aspects of Jesus's attitudes towards others are used to extract ethical principles which, according to Christian vegetarians and vegans, lead one to a vegetarian or vegan lifestyle. Sarah Withrow King summarizes the point like this:

Aware of the suffering and pain experienced by animals raised and killed for food, with a knowledge of the immense waste of natural resources and subsequent impact on both our fellow humans and the rest of creation, and acknowledging that flesh is not a dietary necessity for the vast majority of Western humans, why would we continue to participate in a system that dishonors God’s creation and perpetuates violence on a truly phenomenal scale?

Difficult passages

Luke 24 – Jesus's eating of a fish

Jesus's eating of fish and telling his disciples where to catch fish, before cooking it for them to eat,^{[John 21]} is a common subject in Christian ethical vegetarian and vegan writings. Jesus ate fish and is seen as completely without sin, suggesting that eating fish is not a sin. The Bible does not explicitly state that Jesus ate any meat other than fish, and Webb cites the fact that no lamb is mentioned at the Last Supper as evidence that he did not.

According to Clough and King, the fact that Jesus ate fish (and possibly other meat) only shows that, in some circumstances, it is sometimes permissible to eat some meats, but that practices in the modern, industrialized farming system (such as the mass killing of day-old male chicks from laying hens) make the consumption of meat produced in such farms morally problematic.

Andy Alexis-Baker has appealed to biblical scholarship to argue that biblical passages often need nuanced interpretation, and to guard against a wooden literalism. For example, he cites the work of Gerald O'Collins, SJ, who suggests that differences between the way Luke describes this appearance in Luke 24:41–43 and in Acts 1, and a tension between Luke 24:41–43 and 1 Corinthians 6, preclude us from reading this verse literally.

Vujicic explains this passage by appealing to a so-called synoptic principle.

Acts 10 – Peter's vision

In the tenth chapter of the Acts of the Apostles, there is an account of a vision given to the Apostle Peter. In this vision, Peter is shown a large sheet being lowered from heaven by its four corners. The sheet is said to contain animals of all kinds, and Peter then hears a voice (which he interprets as a command from God) saying, "Get up, Peter; kill and eat".^{[Acts 10:13]} Peter refuses, and the voice says "What God has made clean, you must not call profane".^{[Acts 10:13]}

Christian vegetarians and vegans claim that this passage is not about which animals one may or may not eat, but it is about who the Gospel is for. According to Laura Hobgood-Oster, "The vision, it seems, is not about eating animals; rather it is about extending hospitality to all humans. While animals in sacred texts are often real animals and should be considered as such, in this particular case it seems that in Peter’s vision animals symbolized human categories that exclude other humans from community."

Sarah Withrow King writes that God uses this vision to remind Peter that he is to "remove barriers of fellowship and to reconcile with those from whom we have been separated in order to further the reign of God on earth.... the vision is one of radical inclusion". John Vujicic agrees with King, noting that after receiving the vision, Peter did not eat anything. But, Vujicic writes, "In the sheet were also so called CLEAN animals. Peter could have at least selected some sheep or cattle and killed but he didn’t." According to Vujicic, the reason Peter didn't simply take up and eat a clean animal was because Peter was in fact a vegetarian. Peter is reported as describing himself as a vegetarian in the apocryphal Pseudo-Clementine Homilies.

Mark 7 – Jesus declares all foods clean

Most Christians maintain that Jesus's teaching in Mark 7^{[Mark 7:5–21]} demonstrates that Christians can eat whatever they want, that dietary choices are a matter of "Christian liberty", and that therefore vegetarianism or veganism could never be obligatory for Christians. Christian vegetarians and vegans counter that the point of Jesus's teaching in Mark 7 is that his followers should concern themselves with the status of their heart which "informs our relationship with God, with each other, and the world".

Early Christianity

New Testament

Vegetarianism appears to have been a point of contention within some early Christian circles, notably in Rome. Within the Bible's New Testament, the Apostle Paul states that people of "weak faith" "eat only vegetables",^{[Romans 14:1–4]} although he also warns both meat-eaters and vegetarians to "stop passing judgment on one another" when it comes to food in verse 13 and "[It is] good neither to eat flesh" in verse 21. Paul also said, "The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons. Such teachings come through hypocritical liars, whose consciences have been seared as with a hot iron. They […] order […] to abstain from certain foods".^{[1Timothy 4:1–3]} According to the Christian Vegetarian Association, Paul was not referring to vegetarianism, which they say was not an issue in those times, but to the practice of not eating meat from the meat market because of fear that (like the above issue involving Daniel) it was sacrificed to an idol.^{[1Corinthians 10:19–29]} "Wherefore, if meat [brōma, Strong's #1033, 'anything used as food'] make my brother to offend, I will eat no flesh while the world standeth, lest I make my brother to offend."^{1Corinthians 8:13}

Patristic evidence

In the 4th Century some Jewish Christian groups maintained that Jesus was himself a vegetarian. Epiphanius quotes the Gospel of the Ebionites where Jesus has a confrontation with the high priest. Jesus chastises the leadership saying, "I am come to end the sacrifices and feasts of blood; and if ye cease not offering and eating of flesh and blood, the wrath of God shall not cease from you; even as it came to your fathers in the wilderness, who lusted for flesh, and did sate to their content, and were filled with rottenness, and the plague consumed them."

According to Lightfoot, "the Christianized Essennes […] condemned the slaughter of victims on grounds very different from those alleged in the Epistle of Hebrews, not because they have been superseded by the Atonement, but because they are in their very nature repulsive to God; not because they have ceased to be right, but because they never were right from the beginning".

Other early Christian historical documents observe that many influential Christians during the formative centuries of Christianity were vegetarian, though certainly not all. The Clementine homilies, a second-century work purportedly based on the teachings of the Apostle Peter, states, "The unnatural eating of flesh meats is as polluting as the heathen worship of devils, with its sacrifices and its impure feasts, through participation in it a man becomes a fellow eater with devils." While the Didascalia does not itself endorse vegetarianism, it records a group of individuals who believe they "should not eat flesh, and said that a man must not eat anything that has a soul in it."

Although early Christian vegetarianism appears to have been downplayed in favor of more "modern" Christian culture, the practice of vegetarianism appears to have been very widespread in early Christianity, both in the leadership and among the laity. Origen's work Contra Celsum quotes Celsus commenting vegetarian practices among Christians he had contact with. Although not vegetarian himself and vehemently against the idea that Christians must be vegetarians, Augustine nevertheless wrote that those Christians who "abstain both from flesh and from wine" are "without number".

Churches and movements

Historical developments

Followers of the Gnostic sect known as Catharism practiced pescatarianism as early as the Middle Ages. The Bible Christian Church founded by Reverend William Cowherd in 1809 followed a vegetarian diet. Cowherd was one of the philosophical forerunners of the Vegetarian Society. Cowherd encouraged members to abstain from eating of meat as a form of temperance. Cowherd emphasized that vegetarianism was good for health, whilst eating meat was unnatural and likely to cause aggression. Later he is reputed to have said "If God had meant us to eat meat, then it would have come to us in edible form [as is the ripened fruit]."

Ellen G. White, vegetarian and co-founder of the Seventh-day Adventist Church.

The Seventh-day Adventists present a health message that recommends vegetarianism and expects abstinence from pork, shellfish and other foods proscribed as "unclean" in Leviticus. A number of the founders of the Seventh-day Adventist Church, including Joseph Bates and Ellen White adopted a vegetarian diet during the nineteenth century, and Ellen White reportedly received visions regarding the health benefits of a vegetarian diet. More recently, members of the Seventh-day Adventist Church in California have been involved in research into longevity due to their healthy lifestyle, which includes maintaining a vegetarian diet. This research has been included within a National Geographic article. Another denomination with common origin, the Seventh Day Adventist Reform Movement recommends vegetarianism as a part of fellowship, with many of its members being practicing vegans as well. Typically, however, these sabbatarian pro-vegetarian Christian fellowships do not "require vegetarianism as a test of fellowship."

The Word of Wisdom is a dietary law given to adherents of the Latter Day Saint movement (also known as Mormonism), which states that "flesh also of beasts and of fowls of the air... are to be used sparingly," and that "it is pleasing unto [God] that they should not be used, only in times of winter, or of cold, or famine". Unlike injunctions against tobacco and alcohol, compliance with this part of the Doctrine and Covenants has never been made mandatory by The Church of Jesus Christ of Latter-day Saints (LDS Church), the largest Latter Day Saint denomination. Many LDS Church leaders have expressed their views on the subject of meat, but since Joseph F. Smith became church president in 1901, emphasis on refraining from meat has largely been dropped. An official church publication states, "[m]odern methods of refrigeration now make it possible to preserve meat in any season". As recently as 2012, official church spokesperson Michael Otterson stated "the church has also encouraged limiting meat consumption in favor of grains, fruits and vegetables." Of note is that the LDS Church owns and operates Deseret Ranches in central Florida, which is one of the largest cow-calf operations in the United States.

Some members of the Religious Society of Friends (also known as Quakers) practice vegetarianism or veganism as a reflection of the Peace Testimony, extending non-violence towards animals. Historically, the early vegetarian movement had many Quaker promoters. Some Ranter groups – non-conformist Christian groups that existed in 17th-century England – were vegetarian.

Roman Catholic monastic orders such as the Carthusians and Cistercians follow a pescatarian diet. Carmelites and others following the Rule of St. Albert also maintain a vegetarian diet, although the old and sick are permitted to eat meat according to this rule of life.

Christian anarchists, such as Leo Tolstoy, Ammon Hennacy, and Théodore Monod, extend the Christian principles of compassion and nonviolence through following a vegetarian diet.

Contemporary movements

The Christian Vegetarian Association (CVA) is an international, non-denominational Christian vegetarian organization that promotes responsible stewardship of God's creation through plant-based eating. The CVA produced the 2006 film Honoring God’s Creation.

Sarx is a UK-based organization which aims to "empower Christians to champion the cause of animals and live peacefully with all God’s creatures". Sarx publishes interviews with Christian vegans and vegetarians on its website, and provides people to speak at Churches in the UK on topics such as Christianity and veganism, animal welfare and faith, creation and animals.

CreatureKind is an organization which exists "to encourage Christians to recognize faith-based reasons for caring about the well-being of fellow animal creatures used for food, and to take practical action in response". It was founded by David Clough, Professor of Theological Ethics at the University of Chester, and is directed by Clough and Sarah Withrow King, an American author and deputy directory of the Sider Center at Eastern University. CreatureKind produces a course for churches to do which facilitates church groups to think through how Christians should respond to and treat animals.

Catholic Concern for Animals (CCA) is a charity which calls Catholics "to cherish and care for all of [God's] creation". CCA has for "many years" promoted a vegetarian/vegan diet as a way of caring for creation, in particular animals.

The group Evangelicals for Social Action have suggested that a vegan diet is a way of demonstrating Christian love and compassion to farmed animals, and argue in particular that this is what a consistently pro-life ethic looks like.

Christian Vegetarians and Vegans UK is an organization seeking to promote a vegetarian/vegan lifestyle in the Church in the UK.

Partial fasting and temporary abstinence

Eastern Christianity

During Lent some Christian communities, such as Orthodox Christians in the Middle East, undertake partial fasting eating only one light meal per day. For strict Greek Orthodox Christians and Copts, all meals during this 40-day period are prepared without animal products and are essentially vegan. Unlike veganism, however, abstaining from animal products during Lent is intended to be only temporary and not a permanent way of life.

Eastern Orthodox laity traditionally abstains from animal products on Wednesdays (because, according to Christian tradition, Judas betrayed Jesus on the Wednesday prior to the Crucifixion of Jesus) and Fridays (because Jesus is thought to have been crucified on the subsequent Friday), as well as during the four major fasting periods of the year: Great Lent, the Apostles' Fast, the Dormition Fast and the Nativity Fast. Catholic laity traditionally abstain from animal flesh on Fridays and through the Lenten season leading up to Easter (sometimes being required to do so by law, see fasting and abstinence in the Roman Catholic Church), some also, as a matter of private piety, observe Wednesday abstinence. Fish is not considered proper meat in any case (see pescetarianism, though the Eastern Orthodox allow fish only on days on which the fasting is lessened but meat still not allowed). For these practices, "animal rights" are no motivation and positive environmental or individual health effects only a surplus benefit; the actual reason is to practice mortification and some marginal asceticism.

Oriental Orthodox, Eastern Orthodox, and Eastern Catholic monastics abstain from meat year-round, and many abstain from dairy and seafood as well. Through obedience to the Orthodox Church and its ascetic practices, the Orthodox Christian seeks to rid himself or herself of the passions, or the disposition to sin.

Western Christianity

A Lenten supper prepared according to the diet specified in the Daniel Fast: this particular meal includes black bean spaghetti, quinoa, and mixed vegetables composed of cucumbers, mushrooms, microgreens, arugula, and baby carrots.

In Western Christianity, fasting is observed during the forty-day season of Lent by many communicants of the Catholic Church, Lutheran Churches, Anglican Communion, Methodist Churches and the Western Orthodox Churches to commemorate the fast observed by Christ during his temptation in the desert. While some Western Christians fast during the entire season of Lent, Ash Wednesday and Good Friday are emphasized by Western certain Christian denominations as especially important days of fasting within the Lenten season. In many Western Christian Churches, including those of the Catholic, Methodist and Baptist traditions, certain congregations have committed to undertaking the Daniel Fast during the whole season of Lent, in which believers practice abstinence from meat, lacticinia and alcohol for the entire forty days of the liturgical season.

According to Canon Law, Roman Catholics are required to abstain from meat (defined as all animal flesh and organs, excluding water animals) on Ash Wednesday and all Fridays of Lent including Good Friday. Ash Wednesday and Good Friday are also fast days for Catholics ages 18 to 60, in which one main meal and two half-meals are eaten, with no snacking. Canon Law also obliges Catholics to abstain from meat on the Fridays of the year outside of Lent (excluding certain holy days) unless, with the permission of the local conference of bishops, another penitential act is substituted. Exceptions are allowed for health and necessity like manual labor and not causing offense when being a guest. The restrictions on eating meat on these days is solely as an act of penance and not because of a religious objection to eating meat. In 1966, the United States Conference of Catholic Bishops the conference of bishops has made substitution of a different penitential or charitable act an option for ordinary Fridays in their territory. After previous abolition, the Catholic Bishops' Conference of England and Wales restored the meatless ordinary Friday requirement for their territory effective September, 2011. A popular misconception is that Pope Gregory I (who ruled from 590 to 604, and who is also a canonized saint) declared that rabbits were not meat. This is apparently a corruption of a manuscript in which Saint Gregory of Tours described one person (who was also ill and might not have been Catholic) eating a rabbit fetus during Lent. The rules are widely ignored; a 2016 survey found that only 62% of U.S. Catholics said they avoid meat on Fridays during Lent.

A Handbook for the Discipline of Lent delineates the following Lutheran fasting guidelines:

1. Fast on Ash Wednesday and Good Friday with only one simple meal during the day, usually without meat.
2. Refrain from eating meat (bloody foods) on all Fridays in Lent, substituting fish for example.
3. Eliminate a food or food group for the entire season. Especially consider saving rich and fatty foods for Easter.
4. Consider not eating before receiving Communion in Lent.
5. Abstain from or limit a favorite activity (television, movies, etc.) for the entire season, and spend more time in prayer, Bible study, and reading devotional material.

It is the practice of many Lutherans to abstain from alcohol and meat on the Fridays of Lent; a Black Fast has been historically kept by Lutherans on Good Friday.

In Anglicanism, the Book of Common Prayer prescribes certain days as days for fasting and abstinence from meat, "consisting of the 40 days of Lent, the ember days, the three Rogation days (the Monday to Wednesday following the Sunday after Ascension Day, which is also known as Holy Thursday), and all Fridays in the year (except Christmas, if it falls on a Friday)":

A Table of the Vigils, Fasts, and Days of Abstinence, to be Observed in the Year.

The eves (vigils) before:

The Nativity of our Lord.

The Purification of the Blessed Virgin Mary.

The Annunciation of the Blessed Virgin.

Easter Day.

Ascension Day.

Pentecost.

St. Matthias.

St. John Baptist.

St. Peter.

St. James.

St. Bartholomew.

St. Matthew.

St. Simon and St. Jude.

St. Andrew.

St. Thomas.

All Saints' Day.

Note: if any of these Feast-Days fall upon a Monday, then the Vigil or Fast-Day shall be kept upon the Saturday, and not upon the Sunday next before it.

Days of Fasting, or Abstinence.

I. The Forty Days of Lent.

II. The Ember Days at the Four Seasons, being the Wednesday, Friday, and Saturday after the First Sunday in Lent, the Feast of Pentecost, September 14, and December 13.

III. The Three Rogation Days, being the Monday, Tuesday, and Wednesday, before Holy Thursday, or the Ascension of our Lord.

IV. All the Fridays in the Year, except Christmas Day.

Methodism's principal liturgical book The Sunday Service of the Methodists (put together by John Wesley), as well as The Directions Given to Band Societies (25 December 1744), mandate fasting and abstinence from meat on all Fridays of the year (except Christmas Day, if it falls on a Friday).