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# Axiom

For the algebra software named Axiom, see Axiom computer algebra system. For the 1970s Australian rock music group, see Axiom (band) .

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, axioms are not self-evident truths. They are of two different kinds: logical axioms and non-logical axioms. Axiomatic reasoning is today most widely used in mathematics.

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## 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 formulas which are valid, i.e., formulas that are satisfied by every model (a.k.a. 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.

Now, in order to claim that something is a logical axiom, we must know that it is indeed valid. That is, it might be necessary to offer a proof of its validity (truth) in every model. This might challenge the very classical notion of axiom; this is at least one of the reasons why axioms are not regarded as obviously true or self-evident statements.

Logical axioms, being mere formulas, are devoid of any meaning; but the point is that when they are interpreted in any universe, they will always hold no matter what values are assigned to the variables. Thus, this notion of axiom is perhaps the closest to the intended meaning of the word: that axioms are true, no matter what.

#### Examples

An example, used in virtually every deductive system , is the:

Axiom of equality.

$\forall x (x = x)$

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 "=" (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 that, properly delegating the meaning of "=" to axiomatic set theory.

Another, more interesting example, is that of:

Axiom of 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 formula

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

is valid.

This axiom simply states that if we know $\forall x P(x)\,$ for some property $P\,$, and $t\,$ is particular term in the language (i.e., it stands for a particular object in our structure), then we should be able to claim $P(t)\,$.

Likewise, we have the:

Axiom of 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 formula

$\phi^x_t \to \exists x \phi$

is 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 algebraic 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 fomalized down to the bare language of logical formulas. This turns out to be impossible and proved to be quite a story.

This is the role of non-logical axioms, they simply constitute a starting point in a logical system. Since they are so fundamental in the development of a theory, it is often the case that they are simply referred to as axioms in the mathematical discourse, but again, not in the sense that they are true propositions nor as if they were assumptions claimed to be true. For example, in some groups, the operation of multiplication is commutative; in others it is not.

Thus, an axiom is an elementary basis for a formal logic system that together with the rules of inference define a deductive system.

#### Examples

Arithmetic, Euclidean geometry, linear algebra, real analysis, topology, group theory, set theory, projective geometry, symplectic geometry, von Neumann algebras, ergodic theory, probability, etc. All these theories are based on their respective set of non-logical axioms.

##### Arithmetic

In all this formalism, the Peano axioms constitute the most widely used axiomatization of arithmetic; these are a set of non-logical axioms strong enough to prove several relevant facts of number theory and they allowed Gödel to establish his second incompleteness theorem

The language is $\mathfrak{L}_{NT} = \{0, S\}\,$ where $0\,$ is a constant symbol and $S\,$ is a unary function. The postulates are:

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.

There is a standard structure is $\mathfrak{N} = <\N, 0, S>\,$ where $\N\,$ is the set of natural numbers, $S\,$ is the successor function and $0\,$ is naturally interpreted as the number 0.

##### Geometry

Probably the most famous very early set of axioms is the 4 + 1 postulates of Euclid. This turns out to be incomplete, and many more postulates are necessary to completely characterize his geometry (Hilbert used 23).

"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 real numbers, the standard numbers of "real analysis", are described by the axioms of a complete real closed Archimedean field, which define them uniquely up to isomorphism. 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.

### Deductive systems

The formal issue arises in the need to derive what logicians call a deductive system, which consists of a set Λ of logical axioms, a set Σ of non-logical axioms and a set {(Γ,φ)} of rules of inference. Gödel's completeness theorem establishes that every deductive system with a consistent set of non-logical axioms is complete,

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

i.e., for any statement that is a logical consequence of Σ there actually exists a deduction of the statement from Σ. Again, more simply, anything that is true from a given set of axioms can be proved from those axioms (with reasonable rules of inference).

Note the subtle difference between this and the later and equally celebrated Gödel's first incompleteness theorem, which states that no set of recursive, consistent, non-logical axioms Σ of the Theory of Arithmetic is complete, in the sense that there will always exist a true arithmetic statement φ such that neither φ nor $\lnot\phi$ can be proved (the later is not the same as φ being disproved - it simply means what it says, that there cannot be a deduction from Σ to $\lnot\phi$) from the given set of axioms.

There is thus, in 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 moral is, any fact that we can derive from a set of axioms (logical or non-logical) is not needed as an axiom. Anything that we cannot derive from the axioms and for which we also cannot derive the negation might reasonably be added as an axiom.

### 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.