Rational reconstruction

From Wikipedia, the free encyclopedia

Rational reconstruction is a philosophical term with several distinct meanings. It is found in the work of Jürgen Habermas and Imre Lakatos.

Habermas

For Habermas, rational reconstruction is a philosophical and linguistic method that systematically translates intuitive knowledge of rules into a logical form. In other words, it is an approach to science and philosophy which attempts to put meanings into language properly. It is a type of formal analysis that makes explicit and systematizes the universal and inescapable conditions for the possibility of certain types of phenomena. Rational reconstruction explicates the deep generative structures that give rise to and allow for particular performances, behaviours, and other symbolically pre-structured realities. It is a procedure that is intimately tied to the interpretation of reality, and can deal with an extremely broad range of questions (but not all).

Habermas sees rational reconstruction as a task appropriate for philosophy. This mode of philosophical reflection can be compared to procedures traditionally taken up in philosophy and is concerned with the questions traditionally posed.

Once it has renounced its claim to be a first science or an encyclopedia, philosophy can maintain its status within the scientific system neither by assimilating itself to particular exemplary sciences nor by the exclusive distancing of itself from science in general. Philosophy has to implicate itself in the fallibilistic self-understanding and procedural rationality of the empirical sciences; it may not lay claim to a privileged access to truth, or to a method, an object realm, or even just a style of intuition that is specifically its own. Only thus can philosophy contribute its best to a nonexclusive division of labor, namely, its persistent tenacity in posing questions universalistically, and its procedure of rationally reconstructing the intuitive pretheoretical knowledge of competently speaking, acting and judging subjects. ... This dowry recommends philosophy as an indispensable partner in the collaboration of those who are concerned with [the progress of reason, knowledge and truth]. (p. 38)

— Habermas, Jürgen: (1992) Postmetaphysical Thinking. Cambridge Massachusetts. The MIT Press.

Rational reconstruction is limited to the analysis of symbolically pre-structured realities because it does not deal with the description of reality. So, while the natural sciences generate theoretical knowledge about the general structures of an observable reality, rational reconstructions (sometimes called the reconstructive sciences) generate a theoretical knowledge of the deep structures of a reality accessible only through interpretation. It is hard to point out a phenomenon that is not in some way symbolically pre-structured, that is without some meaning to someone.

While the meaning of any phenomenon must in some way be generated, and the interpretation of symbols is dependent upon intelligence, the bestowal of meaning is not arbitrary. Rational reconstruction attempts to make clear the underlying processes that generate particular meanings. It is concerned with the deep structures of intelligence that generate the knowledge, judgments and actions of subjects as well as the meaning, import and validity of objects. Because of this, those sciences that systematically explicate the intuitive knowledge of competent subjects by reconstructing the pretheoretical know-how of certain human capabilities can be seen as representative of this procedure of rational reconstruction.

Habermas suggests along these lines that "we can distinguish between know-how, the ability of a competent subject who understands how to produce or accomplish something, and know-that, the explicit knowledge of how it is that he is able to do so" (Habermas, 1979). This should be understood in the context of rational reconstruction as two levels of the explication of meaning, two ways of understanding a symbolically pre-structured reality. The difference between the two levels is, in Habermas's terms, the content and the intuitively master rule-consciousness that allows for it, or in other terms, surface structures and deep structures. It is worth quoting Habermas at length to clarify this. Here he is discussing these two levels of analysis as applied to the interpretation of a text:

"...the understanding of content pursues the connections that link the surface structures of an incomprehensible [symbolic] formation with surface structures, of other, familiar formations. Thus, linguistic expressions can be explicated through paraphrase in the same language [etc.]…. If she cannot attain her end in this way, the interpreter may find it necessary to alter her attitude. She then exchanges the attitude of understanding content (directed towards surface structures)…for an attitude in which she focuses on the generative structures of the expressions themselves. The interpreter then attempts to explicate the meaning of a symbolic formation with the help of the rules according to which the author must have produced it….The attitude changes as soon as the interpreter tries not only to apply the intuitive knowledge of speakers but to reconstruct it. She then turns away from the surface structure of the symbolic formation….She attempts instead to peer into the symbolic formation- penetrating through the surface as it were- in order to discover the rules according to which this symbolic formation was produced…. The object of understanding is no longer the content…but the intuitive rule consciousness. (Habermas, 1979)."

Furthermore, following this distinction between surface and deep structures, Habermas views the task of the reconstructive sciences as moving in two directions, horizontal and vertical. The "horizontal" direction seeks to reconstruct fundamental and important competencies, while the "vertical" direction seeks to reconstruct the (genetic) logic of the development of these competencies. So there are surface structures and deep structures related to competencies and sub-competencies, and these are teased apart and reconstructed by engaging two distinct modes of understanding symbolically pre-structured realities.

This should clarify what it is that rational reconstructions seek to accomplish. It should also be said that the results of systematized reconstructions claim to explicate "universal capabilities and not merely the particular competencies of individual groups" (Habermas, 1979). The theoretical claims of such rational reconstructions have the status of general theories of human competencies and behavior. "When the pretheoretical knowledge to be reconstructed expresses a universal capability, a general cognitive, linguistic, or interactive competence (or sub-competence), then what begins as an explication of meaning aims at the reconstruction of species competencies" (Habermas, 1979).

These kinds of rational reconstructions are notably different from the purely philosophical but comparable solutions and methods offered up as definitive answers to such issues in the past. Kant's transcendental analysis sought to deduce the categorical framework that structured and determined experience. Hegel's dialectic sought to systematize the process of knowing, fitting each stage of insight as a defined moment to be subsumed in the development of absolute knowledge. These great systems were conceived as being a-priori, inescapably generative of all experience and knowledge, and they were understood to be factual, to be the truth. Habermas sees rational reconstruction as a similar, but less grandiose, undertaking:

"Marked down in price the transcendental and dialectical modes of justification may still come in handy. All they can fairly be expected to furnish, however, is reconstructive hypotheses for use in empirical settings….[Rational reconstructions are] fallibilistic in orientation, they reject the dubious faith in philosophy's ability to do things single handedly, hoping instead that the success that has for so long eluded it might come from an auspicious matching of different theoretical fragments (Habermas, 1990a)."

Furthermore:

"….[In rational reconstruction] the distinction between drawing on a-priori knowledge and drawing on a-posteriori knowledge becomes blurred. On the one hand, the rule consciousness [i.e. intuitive know-how] of competent subjects is for them an a-priori knowledge; on the other hand, the reconstruction of this calls for inquiries undertaken with empirical [methods] (Habermas, 1979)."

In this respect Habermas sees those theorists whose projects represented a blend of philosophy and scientific methods as important exemplars. He identifies Freud, Durkheim, Mead, Weber, Piaget, Chomsky and Kohlberg as those who "inserted a genuinely philosophical idea like a detonator into a particular context of research…[initiating] paradigms in which a philosophical idea is present in embryo while at the same time empirical, yet universal, questions are being posed" (Habermas, 1990). These theorists approximated the ideal division of labor between philosophy and science that Habermas understands as crucial for progress to be achieved across disciplines; the human sciences in particular represent fertile ground for such cooperation.

Lakatos

See also: Historiographical internalism

Lakatos distinguishes between the "internal" and "external" history of science. Internal history focuses on normative concerns and the reasons scientists can be said to have for accepting or rejecting scientific theories according to some account of the logic of science. External history, on the other hand, focuses on the contingent non-rational factors that influenced the scientific process.

Internalism and externalism

From Wikipedia, the free encyclopedia

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

Internalism and externalism are two opposite ways of integration of explaining various subjects in several areas of philosophy. These include human motivation, knowledge, justification, meaning, and truth. The distinction arises in many areas of debate with similar but distinct meanings. Internal–external distinction is a distinction used in philosophy to divide an ontology into two parts: an internal part concerning observation related to philosophy, and an external part concerning question related to philosophy.

Internalism is the thesis that no fact about the world can provide reasons for action independently of desires and beliefs. Externalism is the thesis that reasons are to be identified with objective features of the world.

Moral philosophy

Motivation

In contemporary moral philosophy, motivational internalism (or moral internalism) is the view that moral convictions (which are not necessarily beliefs, e.g. feelings of moral approval or disapproval) are intrinsically motivating. That is, the motivational internalist believes that there is an internal, necessary connection between one's conviction that X ought to be done and one's motivation to do X. Conversely, the motivational externalist (or moral externalist) claims that there is no necessary internal connection between moral convictions and moral motives. That is, there is no necessary connection between the conviction that X is wrong and the motivational drive not to do X. (The use of these terms has roots in W.D. Falk's (1947) paper "'Ought' and Motivation").

These views in moral psychology have various implications. In particular, if motivational internalism is true, then amorality is unintelligible (and metaphysically impossible). An amoralist is not simply someone who is immoral, rather it is someone who knows what the moral things to do are, yet is not motivated to do them. Such an agent is unintelligible to the motivational internalist, because moral judgments about the right thing to do have built into them corresponding motivations to do those things that are judged by the agent to be the moral things to do. On the other hand, an amoralist is entirely intelligible to the motivational externalist, because the motivational externalist thinks that moral judgments about what is right do not necessitate some motivation to do those things that are judged to be the right thing to do; rather, an independent desire—such as the desire to do the right thing—is required (Brink, 2003), (Rosati, 2006).

Reasons

There is also a distinction in ethics and action theory, largely made popular by Bernard Williams (1979, reprinted in 1981), concerning internal and external reasons for an action. An internal reason is, roughly, something that one has in light of one's own "subjective motivational set"—one's own commitments, desires (or wants), goals, etc. On the other hand, an external reason is something that one has independent of one's subjective motivational set. For example, suppose that Sally is going to drink a glass of poison, because she wants to commit suicide and believes that she can do so by drinking the poison. Sally has an internal reason to drink the poison, because she wants to commit suicide. However, one might say that she has an external reason not to drink the poison because, even though she wants to die, one ought not to kill oneself no matter what—regardless of whether one wants to die.

Some philosophers embrace the existence of both kinds of reason, while others deny the existence of one or the other. For example, Bernard Williams (1981) argues that there are really only internal reasons for action. Such a view is called internalism about reasons (or reasons internalism). Externalism about reasons (or reasons externalism) is the denial of reasons internalism. It is the view that there are external reasons for action; that is, there are reasons for action that one can have even if the action is not part of one's subjective motivational set.

Consider the following situation. Suppose that it's against the moral law to steal from the poor, and Sasha knows this. However, Sasha doesn't desire to follow the moral law, and there is currently a poor person next to him. Is it intelligible to say that Sasha has a reason to follow the moral law right now (to not steal from the poor person next to him), even though he doesn't care to do so? The reasons externalist answers in the affirmative ("Yes, Sasha has a reason not to steal from that poor person."), since he believes that one can have reasons for action even if one does not have the relevant desire. Conversely, the reasons internalist answers the question in the negative ("No, Sasha does not have a reason not to steal from that poor person, though others might."). The reasons internalist claims that external reasons are unintelligible; one has a reason for action only if one has the relevant desire (that is, only internal reasons can be reasons for action). The reasons internalist claims the following: the moral facts are a reason for Sasha's action not to steal from the poor person next to him only if he currently wants to follow the moral law (or if not stealing from the poor person is a way to satisfy his other current goals—that is, part of what Williams calls his "subjective motivational set"). In short, the reasoning behind reasons internalism, according to Williams, is that reasons for action must be able to explain one's action; and only internal reasons can do this.

Epistemology

Justification

Internalism

Two main varieties of epistemic internalism about justification are access internalism and ontological internalism. Access internalists require that a believer must have internal access to the justifier(s) of their belief p in order to be justified in believing p. For the access internalist, justification amounts to something like the believer being aware (or capable of being aware) of certain facts that make her belief in p rational, or them being able to give reasons for her belief in p. At minimum, access internalism requires that the believer have some kind of reflective access or awareness to whatever justifies her belief. Ontological internalism is the view that justification for a belief is established by one's mental states. Ontological internalism can be distinct from access internalism, but the two are often thought to go together since we are generally considered to be capable of having reflective access to mental states.

One popular argument for internalism is known as the 'new evil demon problem'. The new evil demon problem indirectly supports internalism by challenging externalist views of justification, particularly reliabilism. The argument asks us to imagine a subject with beliefs and experiences identical to ours, but the subject is being systematically deceived by a malicious Cartesian demon so that all their beliefs turn out false. In spite of the subject's unfortunate deception, the argument goes, we do not think this subject ceases to be rational in taking things to be as they appear as we do. After all, it is possible that we could be radically deceived in the same way, yet we are still justified in holding most of our beliefs in spite of this possibility. Since reliabilism maintains that one's beliefs are justified via reliable belief-forming processes (where reliable means yielding true beliefs), the subject in the evil demon scenario would not likely have any justified beliefs according to reliabilism because all of their beliefs would be false. Since this result is supposed to clash with our intuitions that the subject is justified in their beliefs in spite of being systematically deceived, some take the new evil demon problem as a reason for rejecting externalist views of justification.

Externalism

Externalist views of justification emerged in epistemology during the late 20th century. Externalist conceptions of justification assert that facts external to the believer can serve as the justification for a belief. According to the externalist, a believer need not have any internal access or cognitive grasp of any reasons or facts which make their belief justified. The externalist's assessment of justification can be contrasted with access internalism, which demands that the believer have internal reflective access to reasons or facts which corroborate their belief in order to be justified in holding it. Externalism, on the other hand, maintains that the justification for someone's belief can come from facts that are entirely external to the agent's subjective awareness.

Alvin Goldman, one of the most well-known proponents of externalism in epistemology, is known for developing a popular form of externalism called reliabilism. In his paper, “What is Justified Belief?” Goldman characterizes the reliabilist conception of justification as such:

"If S’s believing p at t results from a reliable cognitive belief-forming process (or set of processes), then S’s belief in p at t is justified.”

Goldman notes that a reliable belief-forming process is one which generally produces true beliefs.

A unique consequence of reliabilism (and other forms of externalism) is that one can have a justified belief without knowing one is justified (this is not possible under most forms of epistemic internalism). In addition, we do not yet know which cognitive processes are in fact reliable, so anyone who embraces reliabilism must concede that we do not always know whether some of our beliefs are justified (even though there is a fact of the matter).

As a response to skepticism

In responding to skepticism, Hilary Putnam (1982) claims that semantic externalism yields "an argument we can give that shows we are not brains in a vat (BIV). (See also DeRose, 1999.) If semantic externalism is true, then the meaning of a word or sentence is not wholly determined by what individuals think those words mean. For example, semantic externalists maintain that the word "water" referred to the substance whose chemical composition is H₂O even before scientists had discovered that chemical composition. The fact that the substance out in the world we were calling "water" actually had that composition at least partially determined the meaning of the word. One way to use this in a response to skepticism is to apply the same strategy to the terms used in a skeptical argument in the following way (DeRose, 1999):

Either I am a BIV, or I am not a BIV.

If I am not a BIV, then when I say "I am not a BIV", it is true.
If I am a BIV, then, when I say "I am not a BIV", it is true (because "brain" and "vat" would only pick out the brains and vats being simulated, not real brains and real vats).
---

My utterance of "I am not a BIV" is true.

To clarify how this argument is supposed to work: Imagine that there is brain in a vat, and a whole world is being simulated for it. Call the individual who is being deceived "Steve." When Steve is given an experience of walking through a park, semantic externalism allows for his thought, "I am walking through a park" to be true so long as the simulated reality is one in which he is walking through a park. Similarly, what it takes for his thought, "I am a brain in a vat," to be true is for the simulated reality to be one where he is a brain in a vat. But in the simulated reality, he is not a brain in a vat.

Apart from disputes over the success of the argument or the plausibility of the specific type of semantic externalism required for it to work, there is question as to what is gained by defeating the skeptical worry with this strategy. Skeptics can give new skeptical cases that wouldn't be subject to the same response (e.g., one where the person was very recently turned into a brain in a vat, so that their words "brain" and "vat" still pick out real brains and vats, rather than simulated ones). Further, if even brains in vats can correctly believe "I am not a brain in a vat," then the skeptic can still press us on how we know we are not in that situation (though the externalist will point out that it may be difficult for the skeptic to describe that situation).

Another attempt to use externalism to refute skepticism is done by Brueckner and Warfield. It involves the claim that our thoughts are about things, unlike a BIV's thoughts, which cannot be about things (DeRose, 1999).

Semantics

Semantic externalism comes in two varieties, depending on whether meaning is construed cognitively or linguistically. On a cognitive construal, externalism is the thesis that what concepts (or contents) are available to a thinker is determined by their environment, or their relation to their environment. On a linguistic construal, externalism is the thesis that the meaning of a word is environmentally determined. Likewise, one can construe semantic internalism in two ways, as a denial of either of these two theses.

Externalism and internalism in semantics is closely tied to the distinction in philosophy of mind concerning mental content, since the contents of one's thoughts (specifically, intentional mental states) are usually taken to be semantic objects that are truth-evaluable.

See also:

• Linguistic turn for more about the two construals of meaning
• Swamp man thought experiment
• Twin Earth thought experiment

Philosophy of mind

Within the context of the philosophy of mind, externalism is the theory that the contents of at least some of one's mental states are dependent in part on their relationship to the external world or one's environment.

The traditional discussion on externalism was centered around the semantic aspect of mental content. This is by no means the only meaning of externalism now. Externalism is now a broad collection of philosophical views considering all aspects of mental content and activity. There are various forms of externalism that consider either the content or the vehicles of the mind or both. Furthermore, externalism could be limited to cognition, or it could address broader issues of consciousness.

As to the traditional discussion on semantic externalism (often dubbed content externalism), some mental states, such as believing that water is wet, and fearing that the Queen has been insulted, have contents we can capture using 'that' clauses. The content externalist often appeal to observations found as early as Hilary Putnam's seminal essay, "The Meaning of 'Meaning'," (1975). Putnam stated that we can easily imagine pairs of individuals that are microphysical duplicates embedded in different surroundings who use the same words but mean different things when using them.

For example, suppose that Ike and Tina's mothers are identical twins and that Ike and Tina are raised in isolation from one another in indistinguishable environments. When Ike says, "I want my mommy," he expresses a want satisfied only if he is brought to his mommy. If we brought Tina's mommy, Ike might not notice the difference, but he doesn't get what he wants. It seems that what he wants and what he says when he says, "I want my mommy," will be different from what Tina wants and what she says she wants when she says, "I want my mommy."

Externalists say that if we assume competent speakers know what they think, and say what they think, the difference in what these two speakers mean corresponds to a difference in the thoughts of the two speakers that is not (necessarily) reflected by a difference in the internal make up of the speakers or thinkers. They urge us to move from externalism about meaning of the sort Putnam defended to externalism about contentful states of mind. The example pertains to singular terms, but has been extended to cover kind terms as well such as natural kinds (e.g., 'water') and for kinds of artifacts (e.g., 'espresso maker'). There is no general agreement amongst content externalists as to the scope of the thesis.

Philosophers now tend to distinguish between wide content (externalist mental content) and narrow content (anti-externalist mental content). Some, then, align themselves as endorsing one view of content exclusively, or both. For example, Jerry Fodor (1980) argues for narrow content (although he comes to reject that view in his 1995), while David Chalmers (2002) argues for a two dimensional semantics according to which the contents of mental states can have both wide and narrow content.

Critics of the view have questioned the original thought experiments saying that the lessons that Putnam and later writers such as Tyler Burge (1979, 1982) have urged us to draw can be resisted. Frank Jackson and John Searle, for example, have defended internalist accounts of thought content according to which the contents of our thoughts are fixed by descriptions that pick out the individuals and kinds that our thoughts intuitively pertain to the sorts of things that we take them to. In the Ike/Tina example, one might agree that Ike's thoughts pertain to Ike's mother and that Tina's thoughts pertain to Tina's but insist that this is because Ike thinks of that woman as his mother and we can capture this by saying that he thinks of her as 'the mother of the speaker'. This descriptive phrase will pick out one unique woman. Externalists claim this is implausible, as we would have to ascribe to Ike knowledge he wouldn't need to successfully think about or refer to his mother.

Critics have also claimed that content externalists are committed to epistemological absurdities. Suppose that a speaker can have the concept of water we do only if the speaker lives in a world that contains H₂O. It seems this speaker could know a priori that they think that water is wet. This is the thesis of privileged access. It also seems that they could know on the basis of simple thought experiments that they can only think that water is wet if they live in a world that contains water. What would prevent her from putting these together and coming to know a priori that the world contains water? If we should say that no one could possibly know whether water exists a priori, it seems either we cannot know content externalism to be true on the basis of thought experiments or we cannot know what we are thinking without first looking into the world to see what it is like.

As mentioned, content externalism (limited to the semantic aspects) is only one among many other options offered by externalism by and large.

See also:

• Twin Earth thought experiment
• Extended cognition

Historiography of science

See also: Rational reconstruction

Internalism in the historiography of science claims that science is completely distinct from social influences and pure natural science can exist in any society and at any time given the intellectual capacity. Imre Lakatos is a notable proponent of historiographical internalism.

Externalism in the historiography of science is the view that the history of science is due to its social context – the socio-political climate and the surrounding economy determines scientific progress. Thomas Kuhn is a notable proponent of historiographical externalism.

Homochirality

From Wikipedia, the free encyclopedia

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

Homochirality is a uniformity of chirality, or handedness. Objects are chiral when they cannot be superposed on their mirror images. For example, the left and right hands of a human are approximately mirror images of each other but are not their own mirror images, so they are chiral. In biology, 19 of the 20 natural amino acids are homochiral, being L-chiral (left-handed), while sugars are D-chiral (right-handed). Homochirality can also refer to enantiopure substances in which all the constituents are the same enantiomer (a right-handed or left-handed version of an atom or molecule), but some sources discourage this use of the term.

It is unclear whether homochirality has a purpose; however, it appears to be a form of information storage. One suggestion is that it reduces entropy barriers in the formation of large organized molecules. It has been experimentally verified that amino acids form large aggregates in larger abundance from an enantiopure samples of the amino acid than from racemic (enantiomerically mixed) ones.

It is not clear whether homochirality emerged before or after life, and many mechanisms for its origin have been proposed. Some of these models propose three distinct steps: mirror-symmetry breaking creates a minute enantiomeric imbalance, chiral amplification builds on this imbalance, and chiral transmission is the transfer of chirality from one set of molecules to another.

In biology

Amino acids are the building blocks of peptides and enzymes while sugar-peptide chains are the backbone of RNA and DNA. In biological organisms, amino acids appear almost exclusively in the left-handed form (L-amino acids) and sugars in the right-handed form (R-sugars). Since the enzymes catalyze reactions, they enforce homochirality on a great variety of other chemicals, including hormones, toxins, fragrances and food flavors. Glycine is achiral, as are some other non-proteinogenic amino acids that are either achiral (such as dimethylglycine) or of the D enantiomeric form.

Biological organisms easily discriminate between molecules with different chiralities. This can affect physiological reactions such as smell and taste. Carvone, a terpenoid found in essential oils, smells like mint in its L-form and caraway in its R-form. Limonene tastes like citrus when right-handed and pine when left-handed.

Homochirality also affects the response to drugs. Thalidomide, in its left-handed form, cures morning sickness; in its right-handed form, it causes birth defects. Unfortunately, even if a pure left-handed version is administered, some of it can convert to the right-handed form in the patient. Many drugs are available as both a racemic mixture (equal amounts of both chiralities) and an enantiopure drug (only one chirality). Depending on the manufacturing process, enantiopure forms can be more expensive to produce than stereochemical mixtures.

Chiral preferences can also be found at a macroscopic level. Snail shells can be right-turning or left-turning helices, but one form or the other is strongly preferred in a given species. In the edible snail Helix pomatia, only one out of 20,000 is left-helical. The coiling of plants can have a preferred chirality and even the chewing motion of cows has a 10% excess in one direction.

Origins

Unsolved problem in chemistry:

What is the origin of homochirality in living organisms?

Symmetry breaking

Theories for the origin of homochirality in the molecules of life can be classified as deterministic or based on chance depending on their proposed mechanism. If there is a relationship between cause and effect — that is, a specific chiral field or influence causing the mirror symmetry breaking — the theory is classified as deterministic; otherwise it is classified as a theory based on chance (in the sense of randomness) mechanisms.

Another classification for the different theories of the origin of biological homochirality could be made depending on whether life emerged before the enantiodiscrimination step (biotic theories) or afterwards (abiotic theories). Biotic theories claim that homochirality is simply a result of the natural autoamplification process of life—that either the formation of life as preferring one chirality or the other was a chance rare event which happened to occur with the chiralities we observe, or that all chiralities of life emerged rapidly but due to catastrophic events and strong competition, the other unobserved chiral preferences were wiped out by the preponderance and metabolic, enantiomeric enrichment from the 'winning' chirality choices. If this was the case, remains of the extinct chirality sign should be found. Since this is not the case, nowadays biotic theories are no longer supported.

The emergence of chirality consensus as a natural autoamplification process has also been associated with the 2nd law of thermodynamics.

Deterministic theories

Deterministic theories can be divided into two subgroups: if the initial chiral influence took place in a specific space or time location (averaging zero over large enough areas of observation or periods of time), the theory is classified as local deterministic; if the chiral influence is permanent at the time the chiral selection occurred, then it is classified as universal deterministic. The classification groups for local determinist theories and theories based on chance mechanisms can overlap. Even if an external chiral influence produced the initial chiral imbalance in a deterministic way, the outcome sign could be random since the external chiral influence has its enantiomeric counterpart elsewhere.

In deterministic theories, the enantiomeric imbalance is created due to an external chiral field or influence, and the ultimate sign imprinted in biomolecules will be due to it. Deterministic mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the electroweak interaction (via cosmic rays) or asymmetric environments, such as those caused by circularly polarized light, quartz crystals, or the Earth's rotation, β-Radiolysis or the magnetochiral effect. The most accepted universal deterministic theory is the electroweak interaction. Once established, chirality would be selected for.

One supposition is that the discovery of an enantiomeric imbalance in molecules in the Murchison meteorite supports an extraterrestrial origin of homochirality: there is evidence for the existence of circularly polarized light originating from Mie scattering on aligned interstellar dust particles which may trigger the formation of an enantiomeric excess within chiral material in space. Interstellar and near-stellar magnetic fields can align dust particles in this fashion. Another speculation (the Vester-Ulbricht hypothesis) suggests that fundamental chirality of physical processes such as that of the beta decay (see Parity violation) leads to slightly different half-lives of biologically relevant molecules.

Chance theories

Chance theories are based on the assumption that "Absolute asymmetric synthesis, i.e., the formation of enantiomerically enriched products from achiral precursors without the intervention of chiral chemical reagents or catalysts, is in practice unavoidable on statistical grounds alone".

Consider the racemic state as a macroscopic property described by a binomial distribution; the experiment of tossing a coin, where the two possible outcomes are the two enantiomers is a good analogy. The discrete probability distribution ${\displaystyle P_p(n,N)}$ of obtaining n successes out of ${\displaystyle N}$ Bernoulli trials, where the result of each Bernoulli trial occurs with probability ${\displaystyle p}$ and the opposite occurs with probability ${\displaystyle q=(1-p)}$ is given by:

${\displaystyle P_p(n,N)=(\genfrac{}{}{0ex}{}{N}{n})p^n(1-p{)}^{N-n}}$ .

The discrete probability distribution ${\displaystyle P(N/2,N)}$ of having exactly ${\displaystyle N/2}$ molecules of one chirality and ${\displaystyle N/2}$ of the other, is given by:

${\displaystyle P_{1/2}(N/2,N)=(\genfrac{}{}{0ex}{}{N}{N/2}){\left(\frac{1}{2}\right)}^{N/2}{\left(\frac{1}{2}\right)}^{N/2}\approx \sqrt{\frac{2}{\pi N}}}$ .

As in the experiment of tossing a coin, in this case, we assume both events (${\displaystyle L}$ or ${\displaystyle D}$) to be equiprobable, ${\displaystyle p=q=1/2}$. The probability of having exactly the same amount of both enantiomers is inversely proportional to the square root of the total number of molecules ${\displaystyle N}$. For one mol of a racemic compound, ${\displaystyle N=N_A\approx 6.022\cdot {10}^{23}}$ molecules, this probability becomes ${\displaystyle P_{1/2}(N_A/2,N_A)\approx {10}^{-12}}$. The probability of finding the racemic state is so small that we can consider it negligible.

In this scenario, there is a need to amplify the initial stochastic enantiomeric excess through any efficient mechanism of amplification. The most likely path for this amplification step is by asymmetric autocatalysis. An autocatalytic chemical reaction is that in which the reaction product is itself a reactive, in other words, a chemical reaction is autocatalytic if the reaction product is itself the catalyst of the reaction. In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalysing its own production. An initial enantiomeric excess, such as can be produced by polarized light, then allows the more abundant enantiomer to outcompete the other.

Amplification

Theory

Phase portrait of Frank's model: starting from almost everywhere in L-D plane (except L = D line), the system approaches to one of the homochiral states (L=0 or D=0).

In 1953, Charles Frank proposed a model to demonstrate that homochirality is a consequence of autocatalysis. In his model the L and D enantiomers of a chiral molecule are autocatalytically produced from an achiral molecule A

${\displaystyle \begin{array}{r}A+L\stackrel{k_a}{\to }2L,\\ A+D\stackrel{k_a}{\to }2D,\end{array}}$

while suppressing each other through a reaction that he called mutual antagonism

${\displaystyle \begin{array}{r}L+D\stackrel{k_d}{\to }\varnothing .\end{array}}$

In this model the racemic state is unstable in the sense that the slightest enantiomeric excess will be amplified to a completely homochiral state. This can be shown by computing the reaction rates from the law of mass action:

${\displaystyle \begin{array}{rl}\frac{d[\text{L}]}{dt}& =k_a[\text{A}][\text{L}]-k_d[\text{L}][\text{D}]\\ \frac{d[\text{D}]}{dt}& =k_a[\text{A}][\text{D}]-k_d[\text{L}][\text{D}],\end{array}}$

where ${\displaystyle k_a}$ is the rate constant for the autocatalytic reactions, ${\displaystyle k_d}$ is the rate constant for mutual antagonism reaction, and the concentration of A is kept constant for simplicity.

The analytical solutions for are found to be ${\displaystyle [L]/[D]=[L{]}_0/[D{]}_0{e}^{kd([L{]}_0-[D{]}_0)(e^{k_{a}t}-1)}}$ . The ratio ${\displaystyle [L]/[D]}$ increases at a more than exponential rate if ${\displaystyle ([L{]}_0-[D{]}_0)}$ is positive (and vice versa). Every starting conditions different to

${\displaystyle [L{]}_0=[D{]}_0}$ lead to one of the asymptotes ${\displaystyle [L]=0}$ or ${\displaystyle [D]=0}$. Thus the equality of ${\displaystyle [L{]}_0}$ and ${\displaystyle [D{]}_0}$ and so of ${\displaystyle [L]}$ and ${\displaystyle [D]}$ represents a condition of unstable equilibrium, this result depending on the presence of the term representing mutual antagonism.

By defining the enantiomeric excess ${\displaystyle ee}$ as

${\displaystyle ee=\frac{[\text{D}]-[\text{L}]}{[\text{D}]+[\text{L}]},}$

the rate of change of enantiomeric excess can be calculated using chain rule from the rate of change of the concentrations of enantiomers L and D.

${\displaystyle \frac{d(ee)}{dt}=\left(\frac{2k_d[\text{L}][\text{D}]}{[\text{D}]+[\text{L}]}\right)ee.}$

Linear stability analysis of this equation shows that the racemic state ${\displaystyle ee=0}$ is unstable. Starting from almost everywhere in the concentration space, the system evolves to a homochiral state.

It is generally understood that autocatalysis alone does not yield to homochirality, and the presence of the mutually antagonistic relationship between the two enantiomers is necessary for the instability of the racemic mixture. However, recent studies show that homochirality could be achieved from autocatalysis in the absence of the mutually antagonistic relationship, but the underlying mechanism for symmetry-breaking is different.

Experiments

There are several laboratory experiments that demonstrate how a small amount of one enantiomer at the start of a reaction can lead to a large excess of a single enantiomer as the product. For example, the Soai reaction is autocatalytic. If the reaction is started with some of one of the product enantiomers already present, the product acts as an enantioselective catalyst for production of more of that same enantiomer. The initial presence of just 0.2 equivalent one enantiomer can lead to up to 93% enantiomeric excess of the product.

Another study concerns the proline catalyzed aminoxylation of propionaldehyde by nitrosobenzene. In this system, a small enantiomeric excess of catalyst leads to a large enantiomeric excess of product.

Serine octamer clusters are also contenders. These clusters of 8 serine molecules appear in mass spectrometry with an unusual homochiral preference, however there is no evidence that such clusters exist under non-ionizing conditions and amino acid phase behavior is far more prebiotically relevant. The recent observation that partial sublimation of a 10% enantioenriched sample of leucine results in up to 82% enrichment in the sublimate shows that enantioenrichment of amino acids could occur in space. Partial sublimation processes can take place on the surface of meteors where large variations in temperature exist. This finding may have consequences for the development of the Mars Organic Detector scheduled for launch in 2013 which aims to recover trace amounts of amino acids from the Mars surface exactly by a sublimation technique.

A high asymmetric amplification of the enantiomeric excess of sugars are also present in the amino acid catalyzed asymmetric formation of carbohydrates.

One classic study involves an experiment that takes place in the laboratory. When sodium chlorate is allowed to crystallize from water and the collected crystals examined in a polarimeter, each crystal turns out to be chiral and either the L form or the D form. In an ordinary experiment the amount of L crystals collected equals the amount of D crystals (corrected for statistical effects). However, when the sodium chlorate solution is stirred during the crystallization process the crystals are either exclusively L or exclusively D. In 32 consecutive crystallization experiments 14 experiments deliver D-crystals and 18 others L-crystals. The explanation for this symmetry breaking is unclear but is related to autocatalysis taking place in the nucleation process.

In a related experiment, a crystal suspension of a racemic amino acid derivative continuously stirred, results in a 100% crystal phase of one of the enantiomers because the enantiomeric pair is able to equilibrate in solution (compare with dynamic kinetic resolution).

Transmission

Once a significant enantiomeric enrichment has been produced in a system, the transference of chirality through the entire system is customary. This last step is known as the chiral transmission step. Many strategies in asymmetric synthesis are built on chiral transmission. Especially important is the so-called organocatalysis of organic reactions by proline for example in Mannich reactions.

Some proposed models for the transmission of chiral asymmetry are polymerization,epimerization or copolymerization.

Optical resolution in racemic amino acids

There exists no theory elucidating correlations among L-amino acids. If one takes, for example, alanine, which has a small methyl group, and phenylalanine, which has a larger benzyl group, a simple question is in what aspect, L-alanine resembles L-phenylalanine more than D-phenylalanine, and what kind of mechanism causes the selection of all L-amino acids, because it might be possible that alanine was L and phenylalanine was D.

It was reported in 2004 that excess racemic D,L-asparagine (Asn), which spontaneously forms crystals of either isomer during recrystallization, induces asymmetric resolution of a co-existing racemic amino acid such as arginine (Arg), aspartic acid (Asp), glutamine (Gln), histidine (His), leucine (Leu), methionine (Met), phenylalanine (Phe), serine (Ser), valine (Val), tyrosine (Tyr), and tryptophan (Trp). The enantiomeric excess ee = 100 ×(L-D)/(L+D) of these amino acids was correlated almost linearly with that of the inducer, i.e., Asn. When recrystallizations from a mixture of 12 D,L-amino acids (Ala, Asp, Arg, Glu, Gln, His, Leu, Met, Ser, Val, Phe, and Tyr) and excess D,L-Asn were made, all amino acids with the same configuration with Asn were preferentially co-crystallized. It was incidental whether the enrichment took place in L- or D-Asn, however, once the selection was made, the co-existing amino acid with the same configuration at the α-carbon was preferentially involved because of thermodynamic stability in the crystal formation. The maximal ee was reported to be 100%. Based on these results, it is proposed that a mixture of racemic amino acids causes spontaneous and effective optical resolution, even if asymmetric synthesis of a single amino acid does not occur without an aid of an optically active molecule.

This is the first study elucidating reasonably the formation of chirality from racemic amino acids with experimental evidences.

History of term

This term was introduced by Kelvin in 1904, the year that he published his Baltimore Lecture of 1884. Kelvin used the term homochirality as a relationship between two molecules, i.e. two molecules are homochiral if they have the same chirality. Recently, however, homochiral has been used in the same sense as enantiomerically pure. This is permitted in some journals (but not encouraged), its meaning changing into the preference of a process or system for a single optical isomer in a pair of isomers in these journals.