Philosophy of mathematics

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Philosophy of mathematics is that branch of philosophy which attempts to answer questions such as: "why is mathematics useful in describing nature?", "in which sense(s), if any, do mathematical entities such as numbers exist?" and "why and how are mathematical statements true?". Various approaches to answering these questions will be presented in this article.

Contents 1 Relation to philosophy proper 2 Schools of Thought 2.1 Mathematical realism 2.1.1 Platonism 2.1.2 Logicism 2.1.3 Empiricism 2.2 Formalism 2.3 Constructivism and Intuitionism 3 Fictionalism 4 Embodied mind theories 5 Social constructivism or social realism 6 Beyond the "schools" 6.1 Quasi-empiricism 6.2 Action 6.3 Unification 6.4 Ethics 6.5 Aesthetics 6.6 Language 7 External links 7.1 Journals 7.2 Articles 8 References 8.1 Introductions 8.2 Other books

Relation to philosophy proper

Some philosophers of mathematics view their task as giving an account of mathematics and mathematical practice as it stands, as interpretation rather than criticism. Criticisms can, however, have important ramifications for mathematical practice, so the philosophy of mathematics can be of direct interest to working mathematicians, particularly in new fields where the process of peer review of mathematical proofs is not firmly established, raising the probability of an undetected error. Such errors can thus only be reduced by knowing where they are likely to arise. This is a prime concern of the philosophy of mathematics.

More recently some practitioners have also attempted to relate mathematics to general concerns of philosophy: epistemology and ethics in particular. Those concerns are dealt with at the end of this article.

Schools of Thought

The philosophy of mathematics has seen several different schools, distinguished by their pictures of mathematical metaphysics and epistemology. Important questions these schools have considered include, "Why does it work?" and the related--but logically separate -- "Why does mathematics explain the physical world as we see it so well?". Other important questions include "How do we know mathematical truths?" and "What do statements about mathematics mean?"

Three schools, intuitionism, logicism and formalism, emerged around the start of the 20th century in response to the increasingly widespread realisation that mathematics (as it stood), and analysis in particular, did not live up to the standards of certainty and rigour with which it had traditionally been credited. Each school addresses the issues that came to the fore at that time, either attempting to resolve them or claiming that mathematics is not entitled to its status as our most trusted knowledge.

As certainty waned, the original foundations problem in mathematics ("which branch of mathematics is the one from which others are derived?") was restated as an open exploration of foundations of mathematics and shared dependency on certain core concepts like order and set, and then finally as the subfield metamathematics which seems simply to be a mathematical rigorization of the methods of mathematical reasoning.

The schools are addressed separately here and their assumptions explained:

Mathematical realism

Mathematical realism, like realism about any sort of entities, holds that mathematical entities exist independently of the human mind. Thus humans do not invent mathematics, but rather discover it, and any other intelligent beings in the universe would presumably do the same.

Many working mathematicians are mathematical realists; they see themselves as discoverers. Examples are Paul Erdős and Kurt Gödel. Psychological reasons have been given for this preference: it appears to be very hard to preoccupy oneself over long periods of time with the investigation of an entity in whose existence one doesn't firmly believe. Gödel believed in an objective mathematical reality that could be perceived in a manner analogous to sense perception. Certain principles (e.g., for any two mathematical objects, there is a collection of objects consisting of precisely those two objects) could be directly seen to be true, but some conjectures, like the continuum hypothesis, might prove undecidable just on the basis of such principles. Gödel suggested that quasi-empirical methodology could be used to provide sufficient evidence to be able to reasonably assume such a conjecture.

Within realism, there are distinctions depending on what sort of existence one takes mathematical entities to have, and how we know about them.

Platonism

Platonism is the form of realism that suggests that mathematical entities are abstract, have no spatiotemporal or causal properties, and are eternal and unchanging. This is often claimed to be the naive view most people have of numbers. The term Platonism is used because such a view is seen to parallel Plato's belief in a "World of Ideas", an unchanging ultimate reality that the everyday world can only imperfectly approximate. Plato's view probably derives from Pythagoras, and his followers the Pythagoreans, who believed that the world was, quite literally, built up by the numbers. This idea may have even older origins that are unknown to us.

It should be noted that this reading of "Platonism" is rejected by modern Platonist Alain Badiou, who considers the "empiricist" relationship between object and subject (where objects "external" to one's mind act, through the senses, on an "internal" subjective realm) utterly foreign to Platonic thought, according to which this "location" of mathematical entities is irrelevant to their ontological status. Badiou, in fact, identifies mathematics with ontology, considering mathematical discovery to be the scientific investigation of Being qua Being.

The major problem of mathematical platonism is this: precisely where and how do the mathematical entities exist, and how do we know about them? Is there a world, completely separate from our physical one, which is occupied by the mathematical entities? How can we gain access to this separate world and discover truths about the entities?

Gödel's platonism postulates a special kind of mathematical intuition that lets us "perceive" mathematical objects directly. (This view bears resemblances to many things Husserl said about mathematics, and supports Kant's idea that mathematics is synthetic a priori.)

Logicism

Logicism agrees with Gödel that mathematics can be known a priori, but suggests instead that our knowledge of mathematics is just part of our knowledge of logic in general, and is thus analytic, and doesn't require any special faculty. Logic is the proper foundation of mathematics, and all mathematical statements are necessary logical truths. For instance, the statement "If Socrates is a human, and every human is mortal, then Socrates is mortal" is a necessary logical truth.

Gottlob Frege was the founder of logicism. In his seminal Die Grundgesetze der Arithmetik (Basic Laws of Arithmetic) he built up arithmetic from a system of logic with a general principle of comprehension, which he called "Basic Law V" (for concepts F and G, the extension of F equals the extension of G if and only if for all objects a, Fa if and only if Ga), a principle that he took to be acceptable as part of logic.

But Frege's construction was flawed. Russell discovered that Basic Law V is inconsistent (this is Russell's paradox). Frege abandoned his logicist program soon after this, but it was continued by Russell and Whitehead. They attributed the paradox to "vicious circularity" and built up what they called ramified type theory to deal with it. In this system, they were eventually able to build up much of modern mathematics but in an altered, and excessively complex, form (for example, there were different natural numbers in each type, and there were infinitely many types). They also had to make several compromises in order to develop so much of mathematics, such as an "axiom of reducibility". Even Russell said that this axiom did not really belong to logic.

Modern logicists (like Bob Hale, Crispin Wright, and perhaps others) have returned to a program closer to Frege's. They have abandoned Basic Law V in favour of abstraction principles such as Hume's Principle (the number of objects falling under the concept F equals the number of objects falling under the concept G if and only if the extension of F and the extension of G can be put into one-to-one correspondence). Frege required Basic Law V to be able to give an explicit definition of the numbers, but all the properties of numbers can be derived from Hume's Principle. This would not have been enough for Frege because (to paraphrase him) it does not exclude the possibility that the number 3 is in fact Julius Caesar. In addition, many of the weakened principles that they have had to adopt to replace Basic Law V no longer seem so obviously analytic, and thus purely logical.

Empiricism

Empiricism is a form of realism that denies that mathematics can be known a priori at all. It says that we discover mathematical facts by empirical research, just like facts in any of the other sciences. It is not one of the classical three positions advocated in the early 20th century, but primarily arose in the middle of the century. However, an important early proponent of a view like this was John Stuart Mill. Mill's view was widely criticized, because it makes statements like "2+2=4" come out as uncertain, contingent truths, which we can only learn by observing instances of two pairs coming together and forming a quartet.

Contemporary mathematical empiricism, formulated by Quine and Putnam, is primarily supported by the Indispensability Argument: mathematics is indispensable to all empirical sciences, and if we want to believe in the reality of the phenomena described by the sciences, we ought also believe in the reality of those entities required for this description. That is, since physics needs to talk about electrons to say why light bulbs behave as they do, then electrons must exist. Since physics needs to talk about numbers in offering any of its explanations, then numbers must exist. In keeping with Quine and Putnam's overall philosophies, this is a naturalistic argument. It argues for the existence of mathematical entities as the best explanation for experience, thus stripping mathematics of some of its distinctness from the other sciences.

Putnam strongly rejected the term "Platonist" as implying an overly-specific ontology that was not necessary to mathematical practice in any real sense. He advocated a form of "pure realism" that rejected mystical notions of truth and accepted much quasi-empiricism in mathematics. Putnam was involved in coining the term "pure realism" (see below).

The most important criticism of empirical views of mathematics is approximately the same as that raised against Mill. If mathematics is just as empirical as the other sciences, then this suggests that its results are just as fallible as theirs, and just as contingent. In Mill's case the empirical justification comes directly, while in Quine's case it comes indirectly, through the coherence of our scientific theory as a whole. Quine suggests that mathematics seems completely certain because the role it plays in our web of belief is incredibly central, and that it would be extremely difficult for us to revise it, though not impossible.

For a philosophy of mathematics that attempts to overcome some of the shortcomings of Quine and Gödel's approaches by taking aspects of each see Penelope Maddy's Realism in Mathematics. Another example of a realist theory is the embodied mind theory (see below).

Formalism

Formalism holds that mathematical statements may be thought of as statements about the consequences of certain string manipulation rules. For example, in the "game" of Euclidean geometry (which is seen as consisting of some strings called "axioms", and some "rules of inference" to generate new strings from given ones), one can prove that the Pythagorean theorem holds (that is, you can generate the string corresponding to the Pythagorean theorem). Mathematical truths are not about numbers and sets and triangles and the like - in fact, they aren't "about" anything at all!

According to some versions of formalism, the subject matter of mathematics is then literally the written symbols themselves. Then any game is equally good, and one can only play the games, not prove things about them. Unfortunately, this does not solve the metaphysical and epistemological problems (Are the symbols concrete markings on paper, or abstract entities in their own right? If the former, how do we know there are infinitely many of them, and if the latter, then how do we know about them at all?) and does not explain the usefulness of mathematics. This version of formalism is not widely accepted.

A second version of formalism is often known as deductivism. In deductivism, the Pythagorean theorem is not an absolute truth, but a relative one: if you assign meaning to the strings in such a way that the rules of the game become true (ie, true statements are assigned to the axioms and the rules of inference are truth-preserving), then you have to accept the theorem, or, rather, the interpretation you have given it must be a true statement. The same is held to be true for all other mathematical statements. Thus, formalism need not mean that mathematics is nothing more than a meaningless symbolic game. It is usually hoped that there exists some interpretation in which the rules of the game hold. (Compare this position to structuralism.) But it does allow the working mathematician to continue in his or her work and leave such problems to the philosopher or scientist. Many formalists would say that in practice, the axiom systems to be studied will be suggested by the demands of science or other areas of mathematics.

A major early proponent of formalism was David Hilbert, whose goal (Hilbert's program) was a complete and consistent axiomatization of all of mathematics. ("Consistent" here means that no contradictions can be derived from the system.) Hilbert aimed to show the consistency of mathematical systems from the assumption that the "finitary arithmetic" (a subsystem of the usual arithmetic of the positive integers, chosen to be philosophically uncontroversial) was consistent. Hilbert's program was dealt a fatal blow by the second of Gödel's incompleteness theorems, which states that sufficiently expressive consistent axiom systems can never prove their own consistency. Since any such axiom system would contain the finitary arithmetic as a subsystem, Gödel's theorem implied that it would be impossible to prove the system's consistency relative to that (since it would then prove its own consistency, which Gödel had shown was impossible). Thus, in order to show that any controversial body of mathematics is in fact consistent, we need to assume the truth of some even stronger body of mathematics.

Hilbert was initially a deductivist, but, as may be clear from above, he considered certain metamathematical methods to yield intrinsically meaningful results and was a realist with respect to the finitary arithmetic. Later, he held the opinion that there was no other meaningful mathematics whatsoever, regardless of interpretation.

Other formalists, such as Rudolf Carnap, Alfred Tarski and Haskell Curry, considered mathematics to be the investigation of formal axiom systems. Mathematical logicians study formal systems but are just as often realists as they are formalists.

Formalists are usually very tolerant and inviting to new approaches to logic, non-standard number systems, new set theories etc. The more games we study, the better. However, in all three of these examples, motivation is drawn from existing mathematical or philosophical concerns. The "games" are never arbitrarily chosen.

The main critique of formalism is that the actual mathematical ideas that occupy mathematicians are far removed from the minute string manipulation games mentioned above. While published proofs (if correct) could in principle be formulated in terms of these games, the effort required in space and time would be prohibitive (witness Principia Mathematica.) In addition, the rules are certainly not substantial to the initial creation of those proofs. Formalism is also silent to the question of which axiom systems ought to be studied.

Constructivism and Intuitionism

These schools maintain that only mathematical entities which can be explicitly constructed have a claim to existence and should be admitted in mathematical discourse. Mathematics is properly part of the human intuition, and is neither a meaningless game with symbols, nor about some inaccessible realm of platonic entities. Instead, it is about entities that we can create directly through mental activity.

A typical quote comes from Leopold Kronecker: "The natural numbers come from God, everything else is man's work." A major force behind Intuitionism was L.E.J. Brouwer, who rejected the usefulness of formalized logic of any sort for mathematics. Ironically, his student Arend Heyting, postulated an intuitionistic logic, different from the classical Aristotelian logic; this logic does not contain the law of the excluded middle and therefore frowns upon proofs by contradiction. The axiom of choice is also rejected in most intuitionistic set theories, though in some versions it is accepted. Important work was later done by Errett Bishop, who managed to prove versions of the most important theorems in real analysis within this framework.

In Intuitionism, the term "explicit construction" is not cleanly defined, and that has led to criticisms. Attempts have been made to use the concepts of Turing machine or recursive function to fill this gap, leading to the claim that only questions regarding the behavior of finite algorithms are meaningful and should be investigated in mathematics. This has led to the study of the computable numbers, first introduced by Alan Turing.

Intuitionism is the classic example of an anti-realist philosophy of mathematics. (Indeed, Michael Dummett has argued that anti-realism of any sort and intuitionist logic are closely tied together.)

See also: Mathematical constructivism, Mathematical intuitionism

Fictionalism

In 1980, Hartry Field published Science Without Numbers, which turned Quine's indispensability argument on its head. Where Quine suggested that mathematics was indispensable for our best scientific theories, and therefore should be accepted as true, Field suggested that mathematics was dispensable, and therefore should be rejected as false. He did this by giving a complete axiomatization of Newtonian mechanics that didn't reference numbers or functions at all. He started with the "betweenness" axioms of Hilbert geometry to characterize space without coordinatizing it, and then added extra relations between points to do the work standardly done by vector fields.

Having shown how to do science without using mathematics, he rehabilitates mathematics as a kind of "useful fiction". He proves that mathematical physics is a conservative extension of his non-mathematical physics (that is, every physical fact provable in mathematical physics is already provable from his system), so that the mathematics is a reliable process whose physical applications are all true, even though its own statements are false. Thus, when doing mathematics, we can see ourselves as telling a sort of story, talking "as if" numbers existed.

On this account, there are no metaphysical or epistemological problems special to mathematics. The only worries left are the general worries about non-mathematical physics, and about fiction in general.

Embodied mind theories

These theories hold that mathematical thought is a natural outgrowth of the human cognitive apparatus which finds itself in our physical universe. For example, the abstract concept of number springs from the experience of counting discrete objects. It is held that mathematics is not universal and does not exist in any real sense, other than in human brains. Humans construct, but do not discover, mathematics.

The physical universe can thus be seen as the ultimate foundation of mathematics: it guided the evolution of the brain and later determined which questions this brain would find worthy of investigation. However, the human mind has no special claim on "reality" or approaches to it built out of math; If such constructs as Euler's Identity are "true" then they are true as a map of the human mind and cognition, not as a map of anything it "sees".

The effectiveness of mathematics is thus easily explained: mathematics was constructed by the brain in order to be effective in this universe.

The most accessible, famous, and infamous treatment of this perspective is Where Mathematics Comes From, by George Lakoff and Rafael E. Núñez. (Since this book was first published in the year 2000, it may still be one of the only treatments of this perspective.) For more on the science that inspired this perspective, see cognitive science of mathematics.

Social constructivism or social realism

This theory sees mathematics primarily as a social construct, as a product of culture, subject to correction and change. Like the other sciences, mathematics is viewed as an empirical endeavor whose results are constantly evaluated and may be discarded. However, while on an empiricist view the evaluation is some sort of comparison with 'reality', social constructivists emphasize that the direction of mathematical research is dictated by the fashions of the social group performing it or by the needs of the society financing it. However, although such external forces may change the direction of some mathematical research, there are strong internal constraints (the mathematical traditions, methods, problems, meanings and values into which mathematicians are enculturated) that work to conserve the historically defined discipline.

This runs counter to the traditional beliefs of working mathematicians, that mathematics is somehow pure or objective. But social constructivists argue that mathematics is in fact grounded by much uncertainty: as mathematical practice evolves, the status of previous mathematics is cast into doubt, and is corrected to the degree it is required or desired by the current Mathematical Community. This can be seen in the development of analysis from reexamination of the calculus of Leibniz and Newton. They argue further that finished mathematics is often accorded too much status, and folk mathematics not enough, due to an over-belief in axiomatic proof and peer review as practices.

The social nature of mathematics is highlighted in its subcultures. Major discoveries can be made in one branch of mathematics and be relevant to another, yet the relationship goes undiscovered for lack of social contact between mathematicians. Each speciality forms its own epistemic community and often has great difficulty communicating, or motivating the investigation of unifying conjectures that might relate different areas of mathematics. Social constructivists see the process of 'doing mathematics' as actually creating the meaning, while social realists see a deficiency of either human capacity to abstractify, human cognitive bias, or collective intelligence as preventing the comprehension of a 'real' universe of 'mathematical objects'. Social constructivists sometimes reject the search for foundations of mathematics as bound to fail, as pointless or even meaningless. Some social scientists also argue that mathematics is not real or objective at all, but is affected by racism and ethnocentrism. Some of these ideas are close to postmodernism.

Contributions to this school have been made by Imre Lakatos and Thomas Tymoczko, although it is not clear that either would endorse the title. More recently Paul Ernest has explicitly formulated a social constructivist philosophy of mathematics. Some consider the work of Paul Erdős as a whole to have advanced this view (although he personally rejected it) because of his uniquely broad collaborations, which prompted others to see and study "mathematics as a social activity", e.g. via the Erdős number. This strongly influenced work on measuring reputation but has had little impact on mathematics as such.

Beyond the "schools"

Rather than focus on narrow debates about the "true nature" of mathematical truth, or even on practices unique to mathematicians such as the proof, a growing movement from the 1960s to the 1990s began to question the idea of seeking "foundations" or finding any one "right answer" to why mathematics works. The starting point for this was Eugene Wigner's famous 1960 paper The Unreasonable Effectiveness of Mathematics in the Natural Sciences, in which he argued the happy coincidence that mathematics and physics were so well matched, seemed to be "unreasonable" and hard to explain.

The embodied-mind or "cognitive" school and the "social" school were responses to this challenge. But the debates raised were difficult to confine to those:

Quasi-empiricism

One parallel concern that does not actually challenge the schools directly but questions their focus is the notion of quasi-empiricism in mathematics. This grew from the increasingly popular assertion in the late 20th century that no one foundation of mathematics could be ever proven to exist. It is also sometimes called 'postmodernism in mathematics' although that term is considered overloaded by some and insulting by others. It is a very minimal form of social realism/constructivism that accepts that quasi-empirical methods and even sometimes empirical methods can be part of modern mathematical practice.

Such methods have always been part of folk mathematics by which great feats of calculation and measurement are sometimes achieved. Indeed, such methods may be the only notion of "proof" a culture has.

Hilary Putnam argued that any theory of mathematical realism would include quasi-empirical methods. He proposed that an alien species doing mathematics might well rely on quasi-empirical methods primarily, being willing often to forgo rigorous and axiomatic "proofs", and still be "doing mathematics" - at perhaps a somewhat greater risk of failure of their calculations. He laid out a quite detailed argument for this in New Directions (ed. Tymockzo, 1998).

Action

Many practitioners and scholars who are not engaged primarily in proofs have made interesting and important observations about the nature of mathematics:

Judea Pearl claimed that all of mathematics as presently understood was based on an algebra of seeing - and proposed an algebra of doing to complement it - this is a central concern of the philosophy of action and other studies of how "knowing" relates to "doing", or knowledge to action. The most important output of this was new theories of truth, notably those appropriate to activism and grounding empirical methods.

Unification

The notion of a philosophy of mathematics separate from philosophy as such has been criticized as leading to "good mathematicians doing bad philosophy" - few philosophers being able to penetrate mathematical notations and culture to actually relate conventional notions of metaphysics to the more specialized metaphysical notions of the 'schools' above. This may lead to a disconnection in which the mathematicians continue to profess bad and discredited philosophy as a justification for their continued belief in a world-view promoting their work.

Although the social theories and quasi-empiricism, and especially the embodied mind theory, have focused more attention on the epistemology implied by current mathematical practices, they fall far short of actually relating this to ordinary human perception and everyday understandings of knowledge.

Ethics

As well, there is little or no consideration given to the ethics of doing mathematics. In a technological culture, mathematics is seen as an absolute necessity whose value cannot be questioned and whose implications cannot be avoided - even if particular branches have no known purpose, or are considered useful primarily or only to enable conflict, e.g. cryptography, steganography, which are about keeping secrets, or the mathematics involved in optimizing nuclear fission reactions in bombs. While most would accept that physicists bear some moral responsibility for these activities, few have been willing to also criticize mathematicians.

Some of these criticisms have been explored in the sociology of knowledge, but in general mathematics itself has evaded the scrutiny often applied to the sciences of genetics, physics, economics or medicine. Which is interesting in itself, as mathematics is necessary to enable those and other sciences.

Evolutionary psychology for instance has embraced the idea that "the mind is a computer" in the sense of a Turing Machine. What are the implications of adopting an abstraction originating to explain computers formally, to explain the mind?

Aesthetics

Another criticism is that mathematics can be seen very narrowly as the science of measurement and as a vast number of trustworthy shortcuts to reduce the need to measure directly, and simplify calculation. Some of the schools have assigned rather more significance to mathematics than this mere utility -- even seeking sometimes moral guidance, or aesthetics of truth and beauty, in its abstractions. Some consider this a symptom of scientism. Keeping the philosophy of mathematics as a subfield that asks only or primarily 'why does it work?' assuming that it in fact does work in a social or biological sense, as opposed to the narrow sense of physics. It is as inappropriate in this view as having, say, a philosophy of weapons or of war, separate from that of the larger social and species and planetary context of it.

This question is usually rejected by working mathematicians as "irrelevant", but of course they are exactly those people whose aesthetics of proof and of rigour have been already accepted -- they may thus be practicing self-selection of a particular aesthetic, and propagating it with few constraints, especially in those fields where mathematics is not immediately applied to life.

Language

Finally, although many or most mathematicians or philosophers would accept the statement "mathematics is a language", there is little attention paid to the implications of that statement. Linguistics is not applied to discourses or symbol systems of mathematics, that is, mathematics is studied in a markedly different way than other languages. The capacity to acquire mathematics, and competence in it, called numeracy, is seen as separate from literacy and the acquisition of language.

This is because the language of mathematics is far simpler than natural language studied by linguists. However, the methods developed by Gottlob Frege and Alfred Tarski originally for the study of mathematical language have been extended greatly by Tarski's student Richard Montague and other linguists working in formal semantics to show that the distinction between mathematical language and natural language may not be as great as it seems.

See also language education, philosophy of language.

External links

• R.B. Jones' philosophy of mathematics page
• Open Directory Project: Philosophy of Mathematics
• The Philosophy of Real Mathematics Blog

Journals

• Philosophia Mathematica journal
• The Philosophy of Mathematics Education Journal homepage

Articles

• Tymoczko, Thomas (1998 edition) "New Directions in the Philosophy of Mathematics"
• Wigner, Eugene (1960) The Unreasonable Effectiveness of Mathematics in the Natural Sciences, Communications in Pure and Applied Mathematics, vol. 13, No. I, February, New York: John Wiley & Sons.
• Raymond, Eric S., The Utility of Mathematics, (1993)
• Mark Colyvan: Indispensability Arguments in the Philosophy of Mathematics (Edward N. Zalta (ed.), The Stanford Encyclopedia of Philosophy (Fall 2004 Edition))

References

Introductions

• Shapiro, Stewart (2000). Thinking about mathematics: The philosophy of mathematics. Oxford: Oxford University Press.

Other books

• Ernest, Paul (1998). Social constructivism as a philosophy of mathematics. Albany, NY : State University of New York Press.
• Alexandre George (editor), Mathematics and Mind (1994), Oxford University Press, Oxford ISBN 0-19-507929-9
• Maddy, Penelope (1992). Realism in Mathematics.
• Maddy, Penelope (1997). Naturalism in Mathematics.
• Lakoff, George, & Numez, Rafael E. (2000). Where mathematics comes from: How the embodied mind brings mathematics into being. New York: Basic Books.

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