# Axiom

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In epistemology, an axiom is a self-evident truth upon which other knowledge must rest, from which other knowledge is built up. Not all epistemologists agree that any axioms, understood in that sense, exist.

In mathematics, an axiom is not necessarily a self-evident truth but rather, a formal logical expression used in a deduction to yield further results. Mathematics distinguishes two types of axioms: logical axioms and non-logical axioms.

## Etymology

The word axiom comes from the Greek word αξιωμα (axioma), which means that which is deemed worthy or fit or that which is considered self-evident. The word comes from αξιοειν (axioein), meaning to deem worthy, which in turn comes from αξιος (axios), meaning worthy. Among the ancient Greek philosophers an axiom was a claim which could be seen to be true without any need for proof.

## Mathematics

In the field of mathematical logic, a clear distinction is made between two notions of axioms: logical axioms and non-logical axioms.

### Logical axioms

These are certain formulas in a language that are universally valid, that is, formulas that are satisfied by every structure under every variable assignment function. More colloquially, these are statements that are true in any possible universe, under any possible interpretation and with any assignment of values. Usually one takes as logical axioms some minimal set of tautologies that is sufficient for proving all tautologies in the language.

#### Examples

In the propositional calculus it is common to take as logical axioms all formulas of the following forms, where $\phi$, $\psi$, and $\chi$ can be any formulas of the language:

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.

These axiom schemata are also used in the predicate calculus, but additional logical axioms are needed.

Example. Let $\mathfrak{L}\,$ be a first-order language. For each variable $x\,$, the formula

$x = x$

is 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 all what matters, "to be equal") has to be well established first, or a purely formal and syntactical usage of the symbol $=\,$ has to be enforced, and mathematical logic does indeed do that.

Another, more interesting example, is that which provides us with what is known as universal instantiation:

Example. 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 formula

$\forall x. \phi \to \phi^x_t$

is universally valid.

Informally speaking, this example allows us to state that if we know that a certain property $P\,$ holds for every $x\,$ and that if $t\,$ 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. 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 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 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.

Almost every modern mathematical theory 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. This turned out to be impossible and proved to be quite a story (see below).

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 (sometimes referred to as the theory of functions of one real variable), linear algebra, and complex analysis (a.k.a. complex variables), are often introduced non-axiomatically in mostly technical studies, but any rigorous course in these subjects always begins by presenting its axioms.

Geometries such as Euclidean geometry, projective geometry, symplectic geometry. Interestingly one of the results of the fifth Euclidean axiom being a non-logical axiom is that the three angles of a triangle do not by definition add to 180°. Only under the umbrella of Euclidean geometry is this always true.

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 and fields, Galois theory.

This list could be expanded to include most fields of mathematics, including axiomatic set theory, measure theory, ergodic theory, probability, representation theory, and differential geometry.

##### Arithmetic

The Peano axioms are the most widely used axiomatization of arithmetic. They are a set of axioms strong enough to prove many important facts about number theory and they allowed Gödel to establish his famous second incompleteness theorem.

We 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 $\mathfrak{N} = \langle\N, 0, S\rangle\,$ 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. This set of axioms turns out to be incomplete, and many more postulates are necessary to rigorously characterize his geometry (Hilbert used 23).

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 no parallels through a point outside a line exist, that exactly one exists, or that infinitely many exist. These choices give us alternative forms of geometry in which the interior angles of a triangle add up to less than, exactly, or more than a straight line respectively and are known as elliptic, Euclidean, and hyperbolic geometries.

##### Real analysis

The object of study is the real numbers. The real numbers are uniquely picked out (up to isomorphism) by the properties of a complete ordered field. However, expressing these properties as axioms requires use of second-order logic. The Löwenheim-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 $\phi$,

if $\Sigma \models \phi$ then $\Sigma \vdash \phi$

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". Gödel'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 Gödel'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 $\lnot\phi\,$ 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 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, but that the grand parallels between axiomatic systems could be put to good use, as he algebraically solved many classical geometrical problems. 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 we may take as axioms any set of formulas we like, as long as they are not known to be inconsistent.