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An axiom, postulate or assumption is a statement that is taken to be , to serve as a or starting point for further reasoning and arguments. The word comes from the Greek ''axíōma'' () 'that which is thought worthy or fit' or 'that which commends itself as evident.' The term has subtle differences in definition when used in the context of different fields of study. As defined in , an axiom is a statement that is so or well-established, that it is accepted without controversy or question. As used in modern , an axiom is a premise or starting point for reasoning. As used in , the term ''axiom'' is used in two related but distinguishable senses: and . 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., ) are actually substantive assertions about the elements of the domain of a specific mathematical theory (such as ). 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., in ). 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 .

# Etymology

The word ''axiom'' comes from the word (''axíōma''), a from the verb (''axioein''), meaning "to deem worthy", but also "to require", which in turn comes from (''áxios''), meaning "being in balance", and hence "having (the same) value (as)", "worthy", "proper". Among the s an axiom was a claim which could be seen to be self-evidently true without any need for proof. The root meaning of the word ''postulate'' is to "demand"; for instance, demands that one agree that some things can be done (e.g., any two points can be joined by a straight line). Ancient geometers maintained some distinction between axioms and postulates. While commenting on Euclid's books, remarks that " held that this 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." 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 (, rules of inference) was developed by the ancient Greeks, and has become the core principle of modern mathematics. 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 (s, 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 and . The ancient Greeks considered as just one of several s, 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 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 that 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. Aristotle warns that the content of a science cannot be successfully communicated if the learner is in doubt about the truth of the postulates. The classical approach is well-illustrated by , 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 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 with any center and any radius. :# It is true that all s are equal to one another. :# ("") It is true that, if a straight line falling on two straight lines make the on the same side less than two right angles, the two straight lines, if produced indefinitely, on that side on which are the s 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.

## Modern development

A lesson learned by mathematics in the last 150 years is that it is useful to strip the meaning away from the mathematical assertions (axioms, postulates, , theorems) and definitions. One must concede the need for s, or undefined terms or concepts, in any study. Such abstraction or formalization makes mathematical knowledge more general, capable of multiple different meanings, and therefore useful in multiple contexts. , , and were pioneers in this movement. Structuralist mathematics goes further, and develops theories and axioms (e.g. , , , ) without ''any'' particular application in mind. The distinction between an "axiom" and a "postulate" disappears. The postulates of Euclid are profitably motivated by saying that they lead to a great wealth of geometric facts. The truth of these complicated facts rests on the acceptance of the basic hypotheses. However, by throwing out Euclid's fifth postulate, one can get theories that have meaning in wider contexts (e.g., ). As such, one must simply be prepared to use labels such as "line" and "parallel" with greater flexibility. The development of hyperbolic geometry taught mathematicians that it is useful to regard postulates as purely formal statements, and not as facts based on experience. When mathematicians employ the axioms, the intentions are even more abstract. The propositions of field theory do not concern any one particular application; the mathematician now works in complete abstraction. There are many examples of fields; field theory gives correct knowledge about them all. It is not correct to say that the axioms of field theory are "propositions that are regarded as true without proof." Rather, the field axioms are a set of constraints. If any given system of addition and multiplication satisfies these constraints, then one is in a position to instantly know a great deal of extra information about this system. Modern mathematics formalizes its foundations to such an extent that mathematical theories can be regarded as mathematical objects, and mathematics itself can be regarded as a branch of . , , , , and are some of the key figures in this development. Another lesson learned in modern mathematics is to examine purported proofs carefully for hidden assumptions. In the modern understanding, a set of axioms is any of formally stated assertions from which other formally stated assertions follow – by the application of certain well-defined rules. In this view, logic becomes just another formal system. A set of axioms should be ; it should be impossible to derive a contradiction from the axiom. A set of axioms should also be non-redundant; an assertion that can be deduced from other axioms need not be regarded as an axiom. It was the early hope of modern logicians that various branches of mathematics, perhaps all of mathematics, could be derived from a consistent collection of basic axioms. An early success of the formalist program was Hilbert's formalization of , and the related demonstration of the consistency of those axioms. In a wider context, there was an attempt to base all of mathematics on . Here, the emergence of and similar antinomies of raised the possibility that any such system could turn out to be inconsistent. The formalist project suffered a decisive setback, when in 1931 Gödel showed that it is possible, for any sufficiently large set of axioms (, for example) to construct a statement whose truth is independent of that set of axioms. As a , Gödel proved that the consistency of a theory like is an unprovable assertion within the scope of that theory. It is reasonable to believe in the consistency of Peano arithmetic because it is satisfied by the system of s, an but intuitively accessible formal system. However, at present, there is no known way of demonstrating the consistency of the modern for set theory. Furthermore, using techniques of () one can show that the (Cantor) is independent of the Zermelo–Fraenkel axioms. Thus, even this very general set of axioms cannot be regarded as the definitive foundation for mathematics.

## Other sciences

Axioms play a key role not only in mathematics but also in other sciences, notably in . In particular, the monumental work of is essentially based on 's axioms, augmented by a postulate on the non-relation of and the physics taking place in it at any moment. In 1905, Newton's axioms were replaced by those of 's , and later on by those of . Another paper of Albert Einstein and coworkers (see ), almost immediately contradicted by , concerned the interpretation of . This was in 1935. According to Bohr, this new theory should be , whereas according to Einstein it should be . 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, found a theorem, involving complicated optical correlations (see ), 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 got results in favor of Bohr's axioms, not Einstein's. (Bohr's axioms are simply: The theory should be probabilistic in the sense of the .) 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 physical theory needs modification.

# Mathematical logic

In the field of , 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 in a that are , that is, formulas that are by every of values. Usually one takes as logical axioms ''at least'' some minimal set of tautologies that is sufficient for proving all in the language; in the case of more logical axioms than that are required, in order to prove s that are not tautologies in the strict sense.

### =Propositional logic

= In 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 are only "$\neg$" for of the immediately following proposition and "$\to$" for from antecedent to consequent propositions: #$\phi \to \left(\psi \to \phi\right)$ #$\left(\phi \to \left(\psi \to \chi\right)\right) \to \left(\left(\phi \to \psi\right) \to \left(\phi \to \chi\right)\right)$ #$\left(\lnot \phi \to \lnot \psi\right) \to \left(\psi \to \phi\right).$ Each of these patterns is an ', a rule for generating an infinite number of axioms. For example, if $A$, $B$, and $C$ are s, then $A \to \left(B \to A\right)$ and $\left(A \to \lnot B\right) \to \left(C \to \left(A \to \lnot B\right)\right)$ are both instances of axiom schema 1, and hence are axioms. It can be shown that with only these three axiom schemata and ', 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. These axiom schemata are also used in the , but additional logical axioms are needed to include a quantifier in the calculus.

### =First-order logic

=
Axiom of Equality. Let $\mathfrak$ be a . For each variable $x$, the formula
$x = x$
is universally valid.
This means that, for any $x\,,$ 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 , 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 $\mathfrak$, a variable $x$ and a $t$ that is for $x$ in $\phi$, the formula
$\forall x \, \phi \to \phi^x_t$
is universally valid.
Where the symbol $\phi^x_t$ stands for the formula $\phi$ with the term $t$ substituted for $x$. (See .) 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 $P\left(t\right)$. 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''. 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 $\mathfrak$, a variable $x$ and a term $t$ that is substitutable for $x$ in $\phi$, the formula
$\phi^x_t \to \exists x \, \phi$
is 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 s and the s, 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 ). Thus non-logical axioms, unlike logical axioms, are not '. Another name for a non-logical axiom is ''postulate''. Almost every modern starts from a given set of non-logical axioms, and it was thought 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 . 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 , 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 that together with the define a .

### 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 , and are often introduced non-axiomatically, but implicitly or explicitly there is generally an assumption that the axioms being used are the axioms of with choice, abbreviated ZFC, or some very similar system of like , a of ZFC. Sometimes slightly stronger theories such as or set theory with a allowing the use of a is used, but in fact, most mathematicians can actually prove all they need in systems weaker than ZFC, such as . The study of topology in mathematics extends all over through , , , and all the related paraphernalia, such as , . The development of ''abstract algebra'' brought with itself , , , and . This list could be expanded to include most fields of mathematics, including , , , , and .

### =Arithmetic

= The are the most widely used ''axiomatization'' of . They are a set of axioms strong enough to prove many important facts about and they allowed Gödel to establish his famous .Mendelson, "5. The Fixed Point Theorem. Gödel's Incompleteness Theorem" of Ch. 2 We have a language $\mathfrak_ = \$ where $0$ is a constant symbol and $S$ is a and the following axioms: # $\forall x. \lnot \left(Sx = 0\right)$ # $\forall x. \forall y. \left(Sx = Sy \to x = y\right)$ # $\left(\phi\left(0\right) \land \forall x.\,\left(\phi\left(x\right) \to \phi\left(Sx\right)\right)\right) \to \forall x.\phi\left(x\right)$ for any $\mathfrak_$ formula $\phi$ with one free variable. The standard structure is $\mathfrak = \langle\N, 0, S\rangle$ where $\N$ is the set of natural numbers, $S$ is the and $0$ is naturally interpreted as the number 0.

### =Euclidean geometry

= Probably the oldest, and most famous, list of axioms are the 4 + 1 of . The axioms are referred to as "4 + 1" because for nearly two millennia the ("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. 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 s of a add up to exactly 180 degrees or less, respectively, and are known as Euclidean and geometries. If one also removes the second postulate ("a line can be extended indefinitely") then 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 the study are within the domain of . The real numbers are uniquely picked out (up to ) 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 the use of . The s tell us that if we restrict ourselves to , 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 .

## Role in mathematical logic

### Deductive systems and completeness

A system consists of a set $\Lambda$ of logical axioms, a set $\Sigma$ of non-logical axioms, and a set $\$ 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 $\phi$,
$\text\Sigma \models \phi\text\Sigma \vdash \phi$
that 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". 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 , 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 $\lnot\phi$ 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 s regarded as a model of , 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 made elaborate efforts to derive them from traditional arithmetic. 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 was born. In the modern view, axioms may be any set of formulas, as long as they are not known to be inconsistent.

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