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{{about|axioms as used in logic and in mathematics}}{{Redirect|Axiomatic}}{{redirect|Axioms|the academic journal|Axioms (journal)}}{{redirect|Postulation|the term in algebraic geometry|Postulation (algebraic geometry)}}{{short description|Statement that is taken to be true}}{{Use dmy dates|date=June 2013}}An axiom or postulate is a statement that is taken to be true, to serve as a premise or starting point for further reasoning and arguments. The word comes from the Greek axÃ­Åma (}}) 'that which is thought worthy or fit' or 'that which commends itself as evident.'Cf. axiom, n., etymology. Oxford English Dictionary, accessed 2012-04-28.Oxford American College Dictionary: "n. a statement or proposition that is regarded as being established, accepted, or self-evidently true. ORIGIN: late 15th cent.: ultimately from Greek axiÅma 'what is thought fitting,' from axios 'worthy.' HighBeam{{dead link|date=February 2019|bot=medic}}{{cbignore|bot=medic}} {{subscription}}The term has subtle differences in definition when used in the context of different fields of study. As defined in classic philosophy, an axiom is a statement that is so evident or well-established, that it is accepted without controversy or question."A proposition that commends itself to general acceptance; a well-established or universally conceded principle; a maxim, rule, law" axiom, n., definition 1a. Oxford English Dictionary Online, accessed 2012-04-28. Cf. Aristotle, Posterior Analytics I.2.72a18-b4. As used in modern logic, an axiom is a premise or starting point for reasoning."A proposition (whether true or false)" axiom, n., definition 2. Oxford English Dictionary Online, accessed 2012-04-28.
As used in mathematics, the term axiom is used in two related but distinguishable senses: "logical axioms" and "non-logical axioms". Logical axioms are usually statements that are taken to be true within the system of logic they define and are often shown in symbolic form (e.g., (A and B) implies A), while non-logical axioms (e.g., {{nowrap|1= a + b = b + a}}) are actually substantive assertions about the elements of the domain of a specific mathematical theory (such as arithmetic). When used in the latter sense, "axiom", "postulate", and "assumption" may be used interchangeably. In most cases, a non-logical axiom is simply a formal logical expression used in deduction to build a mathematical theory, and might or might not be self-evident in nature (e.g., parallel postulate in Euclidean geometry).WEB,weblink The Definitive Glossary of Higher Mathematical Jargon, 2019-08-01, Math Vault, en-US, live, 2019-10-19, To axiomatize a system of knowledge is to show that its claims can be derived from a small, well-understood set of sentences (the axioms), and there may be multiple ways to axiomatize a given mathematical domain.Any axiom is a statement that serves as a starting point from which other statements are logically derived. Whether it is meaningful (and, if so, what it means) for an axiom to be "true" is a subject of debate in the philosophy of mathematics.See for example JOURNAL, Penelope, Maddy, Journal of Symbolic Logic, Believing the Axioms, I, 53, 2, Jun 1988, 481â€“511, 10.2307/2274520, for a realist view.

Etymology

The word axiom comes from the Greek word (axÃ­Åma), a verbal noun from the verb (axioein), meaning "to deem worthy", but also "to require", which in turn comes from (Ã¡xios), meaning "being in balance", and hence "having (the same) value (as)", "worthy", "proper". Among the ancient Greek philosophers an axiom was a claim which could be seen to be self-evidently true without any need for proof.WEB,weblink Axiom â€” Powszechna Encyklopedia Filozofii, Polskie Towarzystwo Tomasza z Akwinu, live, The root meaning of the word postulate is to "demand"; for instance, Euclid demands that one agree that some things can be done (e.g., any two points can be joined by a straight line).Wolff, P. Breakthroughs in Mathematics, 1963, New York: New American Library, pp 47â€“48Ancient geometers maintained some distinction between axioms and postulates. While commenting on Euclid's books, Proclus remarks that, "Geminus held that this [4th] Postulate should not be classed as a postulate but as an axiom, since it does not, like the first three Postulates, assert the possibility of some construction but expresses an essential property."Heath, T. 1956. The Thirteen Books of Euclid's Elements. New York: Dover. p 200 Boethius translated 'postulate' as petitio and called the axioms notiones communes but in later manuscripts this usage was not always strictly kept.

Historical development

Early Greeks

The logico-deductive method whereby conclusions (new knowledge) follow from premises (old knowledge) through the application of sound arguments (syllogisms, rules of inference) was developed by the ancient Greeks, and has become the core principle of modern mathematics. Tautologies excluded, nothing can be deduced if nothing is assumed. Axioms and postulates are thus the basic assumptions underlying a given body of deductive knowledge. They are accepted without demonstration. All other assertions (theorems, in the case of mathematics) must be proven with the aid of these basic assumptions. However, the interpretation of mathematical knowledge has changed from ancient times to the modern, and consequently the terms axiom and postulate hold a slightly different meaning for the present day mathematician, than they did for Aristotle and Euclid.The ancient Greeks considered geometry as just one of several sciences, and held the theorems of geometry on par with scientific facts. As such, they developed and used the logico-deductive method as a means of avoiding error, and for structuring and communicating knowledge. Aristotle's posterior analytics is a definitive exposition of the classical view.An "axiom", in classical terminology, referred to a self-evident assumption common to many branches of science. A good example would be the assertion that When an equal amount is taken from equals, an equal amount results.At the foundation of the various sciences lay certain additional hypotheses which were accepted without proof. Such a hypothesis was termed a postulate. While the axioms were common to many sciences, the postulates of each particular science were different. Their validity had to be established by means of real-world experience. Indeed, Aristotle warns that the content of a science cannot be successfully communicated, if the learner is in doubt about the truth of the postulates.Aristotle, Metaphysics Bk IV, Chapter 3, 1005b "Physics also is a kind of Wisdom, but it is not the first kind. â€“ And the attempts of some of those who discuss the terms on which truth should be accepted, are due to want of training in logic; for they should know these things already when they come to a special study, and not be inquiring into them while they are listening to lectures on it." W.D. Ross translation, in The Basic Works of Aristotle, ed. Richard McKeon, (Random House, New York, 1941)The classical approach is well-illustrated by Euclid's Elements, where a list of postulates is given (common-sensical geometric facts drawn from our experience), followed by a list of "common notions" (very basic, self-evident assertions).
;Postulates
# It is possible to draw a straight line from any point to any other point. # It is possible to extend a line segment continuously in both directions. # It is possible to describe a circle with any center and any radius. # It is true that all right angles are equal to one another. # ("Parallel postulate") It is true that, if a straight line falling on two straight lines make the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, intersect on that side on which are the angles less than the two right angles.
;Common notions:
# Things which are equal to the same thing are also equal to one another. # If equals are added to equals, the wholes are equal. # If equals are subtracted from equals, the remainders are equal. # Things which coincide with one another are equal to one another. # The whole is greater than the part.

Other sciences

Axioms play a key role not only in mathematics, but also in other sciences, notably in theoretical physics. In particular, the monumental work of Isaac Newton is essentially based on Euclid's axioms, augmented by a postulate on the non-relation of spacetime and the physics taking place in it at any moment.In 1905, Newton's axioms were replaced by those of Albert Einstein's special relativity, and later on by those of general relativity.Another paper of Albert Einstein and coworkers (see EPR paradox), almost immediately contradicted by Niels Bohr, concerned the interpretation of quantum mechanics. This was in 1935. According to Bohr, this new theory should be probabilistic, whereas according to Einstein it should be deterministic. Notably, the underlying quantum mechanical theory, i.e. the set of "theorems" derived by it, seemed to be identical. Einstein even assumed that it would be sufficient to add to quantum mechanics "hidden variables" to enforce determinism. However, thirty years later, in 1964, John Bell found a theorem, involving complicated optical correlations (see Bell inequalities), which yielded measurably different results using Einstein's axioms compared to using Bohr's axioms. And it took roughly another twenty years until an experiment of Alain Aspect got results in favour of Bohr's axioms, not Einstein's. (Bohr's axioms are simply: The theory should be probabilistic in the sense of the Copenhagen interpretation.)As a consequence, it is not necessary to explicitly cite Einstein's axioms, the more so since they concern subtle points on the "reality" and "locality" of experiments.Regardless, the role of axioms in mathematics and in the above-mentioned sciences is different. In mathematics one neither "proves" nor "disproves" an axiom for a set of theorems; the point is simply that in the conceptual realm identified by the axioms, the theorems logically follow. In contrast, in physics a comparison with experiments always makes sense, since a falsified physical theory needs modification.

Mathematical logic

In the field of mathematical logic, a clear distinction is made between two notions of axioms: logical and non-logical (somewhat similar to the ancient distinction between "axioms" and "postulates" respectively).

Logical axioms

These are certain formulas in a formal language that are universally valid, that is, formulas that are satisfied by every assignment of values. Usually one takes as logical axioms at least some minimal set of tautologies that is sufficient for proving all tautologies in the language; in the case of predicate logic more logical axioms than that are required, in order to prove logical truths that are not tautologies in the strict sense.

Propositional logic

In propositional logic it is common to take as logical axioms all formulae of the following forms, where phi, chi, and psi can be any formulae of the language and where the included primitive connectives are only "neg" for negation of the immediately following proposition and "to" for implication from antecedent to consequent propositions:
1. phi to (psi to phi)
2. (phi to (psi to chi)) to ((phi to psi) to (phi to chi))
3. (lnot phi to lnot psi) to (psi to phi).
Each of these patterns is an axiom schema, a rule for generating an infinite number of axioms. For example, if A, B, and C are propositional variables, then A to (B to A) and (A to lnot B) to (C to (A to lnot B)) are both instances of axiom schema 1, and hence are axioms. It can be shown that with only these three axiom schemata and modus ponens, one can prove all tautologies of the propositional calculus. It can also be shown that no pair of these schemata is sufficient for proving all tautologies with modus ponens.Other axiom schemata involving the same or different sets of primitive connectives can be alternatively constructed.Mendelson, "6. Other Axiomatizations" of Ch. 1These axiom schemata are also used in the predicate calculus, but additional logical axioms are needed to include a quantifier in the calculus.Mendelson, "3. First-Order Theories" of Ch. 2

First-order logic

Axiom of Equality. Let mathfrak{L} be a first-order language. For each variable x, the formulax = xis universally valid.This means that, for any variable symbol x,, the formula x = x can be regarded as an axiom. Also, in this example, for this not to fall into vagueness and a never-ending series of "primitive notions", either a precise notion of what we mean by x = x (or, for that matter, "to be equal") has to be well established first, or a purely formal and syntactical usage of the symbol = has to be enforced, only regarding it as a string and only a string of symbols, and mathematical logic does indeed do that.Another, more interesting example axiom scheme, is that which provides us with what is known as Universal Instantiation:Axiom scheme for Universal Instantiation. Given a formula phi in a first-order language mathfrak{L}, a variable x and a term t that is substitutable for x in phi, the formulaforall x , phi to phi^x_tis universally valid.Where the symbol phi^x_t stands for the formula phi with the term t substituted for x. (See Substitution of variables.) In informal terms, this example allows us to state that, if we know that a certain property P holds for every x and that t stands for a particular object in our structure, then we should be able to claim P(t). Again, we are claiming that the formula forall x phi to phi^x_t is valid, that is, we must be able to give a "proof" of this fact, or more properly speaking, a metaproof. Actually, these examples are metatheorems of our theory of mathematical logic since we are dealing with the very concept of proof itself. Aside from this, we can also have Existential Generalization:Axiom scheme for Existential Generalization. Given a formula phi in a first-order language mathfrak{L}, a variable x and a term t that is substitutable for x in phi, the formulaphi^x_t to exists x , phiis universally valid.

Non-logical axioms

Non-logical axioms are formulas that play the role of theory-specific assumptions. Reasoning about two different structures, for example the natural numbers and the integers, may involve the same logical axioms; the non-logical axioms aim to capture what is special about a particular structure (or set of structures, such as groups). Thus non-logical axioms, unlike logical axioms, are not tautologies. Another name for a non-logical axiom is postulate.Mendelson, "3. First-Order Theories: Proper Axioms" of Ch. 2Almost every modern mathematical theory starts from a given set of non-logical axioms, and it was{{Explain|date=June 2019|reason=use of past tense without explanation of change}} thought{{Citation needed|date=July 2011}} that in principle every theory could be axiomatized in this way and formalized down to the bare language of logical formulas.Non-logical axioms are often simply referred to as axioms in mathematical discourse. This does not mean that it is claimed that they are true in some absolute sense. For example, in some groups, the group operation is commutative, and this can be asserted with the introduction of an additional axiom, but without this axiom we can do quite well developing (the more general) group theory, and we can even take its negation as an axiom for the study of non-commutative groups.Thus, an axiom is an elementary basis for a formal logic system that together with the rules of inference define a deductive system.

Examples

This section gives examples of mathematical theories that are developed entirely from a set of non-logical axioms (axioms, henceforth). A rigorous treatment of any of these topics begins with a specification of these axioms.Basic theories, such as arithmetic, real analysis and complex analysis are often introduced non-axiomatically, but implicitly or explicitly there is generally an assumption that the axioms being used are the axioms of Zermeloâ€“Fraenkel set theory with choice, abbreviated ZFC, or some very similar system of axiomatic set theory like Von Neumannâ€“Bernaysâ€“GÃ¶del set theory, a conservative extension of ZFC. Sometimes slightly stronger theories such as Morseâ€“Kelley set theory or set theory with a strongly inaccessible cardinal allowing the use of a Grothendieck universe are used, but in fact most mathematicians can actually prove all they need in systems weaker than ZFC, such as second-order arithmetic.{{citation needed|reason=This claim should include a citation |date=April 2016}}The study of topology in mathematics extends all over through point set topology, algebraic topology, differential topology, and all the related paraphernalia, such as homology theory, homotopy theory. The development of abstract algebra brought with itself group theory, rings, fields, and Galois theory.This list could be expanded to include most fields of mathematics, including measure theory, ergodic theory, probability, representation theory, and differential geometry.

Arithmetic

The Peano axioms are the most widely used axiomatization of first-order arithmetic. They are a set of axioms strong enough to prove many important facts about number theory and they allowed GÃ¶del to establish his famous second incompleteness theorem.Mendelson, "5. The Fixed Point Theorem. GÃ¶del's Incompleteness Theorem" of Ch. 2We have a language mathfrak{L}_{NT} = {0, S} where 0 is a constant symbol and S is a unary function and the following axioms:
1. forall x. lnot (Sx = 0)
2. forall x. forall y. (Sx = Sy to x = y)
3. (phi(0) land forall x.,(phi(x) to phi(Sx))) to forall x.phi(x) for any mathfrak{L}_{NT} formula phi with one free variable.
The standard structure is mathfrak{N} = langleN, 0, Srangle where N is the set of natural numbers, S is the successor function and 0 is naturally interpreted as the number 0.

Euclidean geometry

Probably the oldest, and most famous, list of axioms are the 4 + 1 Euclid's postulates of plane geometry. The axioms are referred to as "4 + 1" because for nearly two millennia the fifth (parallel) postulate ("through a point outside a line there is exactly one parallel") was suspected of being derivable from the first four. Ultimately, the fifth postulate was found to be independent of the first four. Indeed, one can assume that exactly one parallel through a point outside a line exists, or that infinitely many exist. This choice gives us two alternative forms of geometry in which the interior angles of a triangle add up to exactly 180 degrees or less, respectively, and are known as Euclidean and hyperbolic geometries. If one also removes the second postulate ("a line can be extended indefinitely") then elliptic geometry arises, where there is no parallel through a point outside a line, and in which the interior angles of a triangle add up to more than 180 degrees.

Real analysis

The objectives of study are within the domain of real numbers. The real numbers are uniquely picked out (up to isomorphism) by the properties of a Dedekind complete ordered field, meaning that any nonempty set of real numbers with an upper bound has a least upper bound. However, expressing these properties as axioms requires use of second-order logic. The LÃ¶wenheimâ€“Skolem theorems tell us that if we restrict ourselves to first-order logic, any axiom system for the reals admits other models, including both models that are smaller than the reals and models that are larger. Some of the latter are studied in non-standard analysis.

Role in mathematical logic

Deductive systems and completeness

A deductive system consists of a set Lambda of logical axioms, a set Sigma of non-logical axioms, and a set {(Gamma, phi)} of rules of inference. A desirable property of a deductive system is that it be complete. A system is said to be complete if, for all formulas phi,text{if }Sigma models phitext{ then }Sigma vdash phithat is, for any statement that is a logical consequence of Sigma there actually exists a deduction of the statement from Sigma. This is sometimes expressed as "everything that is true is provable", but it must be understood that "true" here means "made true by the set of axioms", and not, for example, "true in the intended interpretation". GÃ¶del's completeness theorem establishes the completeness of a certain commonly used type of deductive system.Note that "completeness" has a different meaning here than it does in the context of GÃ¶del's first incompleteness theorem, which states that no recursive, consistent set of non-logical axioms Sigma of the Theory of Arithmetic is complete, in the sense that there will always exist an arithmetic statement phi such that neither phi nor lnotphi can be proved from the given set of axioms.There is thus, on the one hand, the notion of completeness of a deductive system and on the other hand that of completeness of a set of non-logical axioms. The completeness theorem and the incompleteness theorem, despite their names, do not contradict one another.

Further discussion

Early mathematicians regarded axiomatic geometry as a model of physical space, and obviously there could only be one such model. The idea that alternative mathematical systems might exist was very troubling to mathematicians of the 19th century and the developers of systems such as Boolean algebra made elaborate efforts to derive them from traditional arithmetic. Galois showed just before his untimely death that these efforts were largely wasted. Ultimately, the abstract parallels between algebraic systems were seen to be more important than the details and modern algebra was born. In the modern view axioms may be any set of formulas, as long as they are not known to be inconsistent.

References

{{reflist}}

• Mendelson, Elliot (1987). Introduction to mathematical logic. Belmont, California: Wadsworth & Brooks. {{ISBN|0-534-06624-0}}
• BOOK, Wilson, John Cook, John Cook Wilson, s:On an Evolutionist Theory of Axioms, On an Evolutionist Theory of Axioms]], 1889, Clarendon Press, Oxford,

{{Wiktionary|axiom|given}}{{EB1911 poster|Axiom}}
{{Mathematical logic}}

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