In classical deductive logic, a **consistent** theory is one that does not lead to a logical contradiction.^{[1]} The lack of contradiction can be defined in either semantic or syntactic terms. The semantic definition states that a theory is consistent if it has a model, i.e., there exists an interpretation under which all formulas in the theory are true. This is the sense used in traditional Aristotelian logic, although in contemporary mathematical logic the term *satisfiable* is used instead. The syntactic definition states a theory is consistent if there is no formula such that both and its negation are elements of the set of consequences of . Let be a set of closed sentences (informally "axioms") and the set of closed sentences provable from under some (specified, possibly implicitly) formal deductive system. The set of axioms is **consistent** when there is no formula such that and .^{[2]}

If there exists a deductive system for which these semantic and syntactic definitions are equivalent for any theory formulated in a particular deductive logic, the logic is called **complete**.^{[citation needed]} The completeness of the sentential calculus was proved by Paul Bernays in 1918^{[citation needed]}^{[3]} and Emil Post in 1921,^{[4]} while the completeness of predicate calculus was proved by Kurt Gödel in 1930,^{[5]} and consistency proofs for arithmetics restricted with respect to the induction axiom schema were proved by Ackermann (1924), von Neumann (1927) and Herbrand (1931).^{[6]} Stronger logics, such as second-order logic, are not complete.

A **consistency proof** is a mathematical proof that a particular theory is consistent.^{[7]} The early development of mathematical proof theory was driven by the desire to provide finitary consistency proofs for all of mathematics as part of Hilbert's program. Hilbert's program was strongly impacted by the incompleteness theorems, which showed that sufficiently strong proof theories cannot prove their consistency (provided that they are consistent).

Although consistency can be proved using model theory, it is often done in a purely syntactical way, without any need to reference some model of the logic. The cut-elimination (or equivalently the normalization of the underlying calculus if there is one) implies the consistency of the calculus: since there is no cut-free proof of falsity, there is no contradiction in general.

In theories of arithmetic, such as Peano arithmetic, there is an intricate relationship between the consistency of the theory and its completeness. A theory is complete if, for every formula φ in its language, at least one of φ or ¬φ is a logical consequence of the theory.

Presburger arithmetic is an axiom system for the natural numbers under addition. It is both consistent and complete.

Gödel's incompleteness theorems show that any sufficiently strong recursively enumerable theory of arithmetic cannot be both complete and consistent. Gödel's theorem applies to the theories of Peano arithmetic (PA) and primitive recursive arithmetic (PRA), but not to Presburger arithmetic.

Moreover, Gödel's second incompleteness theorem shows that the consistency of sufficiently strong recursively enumerable theories of arithmetic can be tested in a particular way. Such a theory is consistent if and only if it does *not* prove a particular sentence, called the Gödel sentence of the theory, which is a formalized statement of the claim that the theory is indeed consistent. Thus the consistency of a sufficiently strong, recursively enumerable, consistent theory of arithmetic can never be proven in that system itself. The same result is true for recursively enumerable theories that can describe a strong enough fragment of arithmetic—including set theories such as Zermelo–Fraenkel set theory (ZF). These set theories cannot prove their own Gödel sentence—provided that they are consistent, which is generally believed.

Because consistency of ZF is not provable in ZF, the weaker notion **relative consistency** is interesting in set theory (and in other sufficiently expressive axiomatic systems). If *T* is a theory and *A* is an additional axiom, *T* + *A* is said to be consistent relative to *T* (or simply that *A* is consistent with *T*) if it can be proved that
if *T* is consistent then *T* + *A* is consistent. If both *A* and ¬*A* are consistent with *T*, then *A* is said to be independent of *T*.

In the following context of mathematical logic, the turnstile symbol means "provable from". That is, reads: *b* is provable from *a* (in some specified formal system).

- A set of formulas in first-order logic is
**consistent**(written ) if there is no formula such that and . Otherwise is**inconsistent**(written ). - is said to be
**simply consistent**if for no formula of , both and the negation of are theorems of .^{[clarification needed]} - is said to be
**absolutely consistent**or**Post consistent**if at least one formula in the language of is not a theorem of . - is said to be
**maximally consistent**if is consistent and for every formula , implies . - is said to
**contain witnesses**if for every formula of the form there exists a term such that , where denotes the substitution of each in by a ; see also First-order logic.^{[citation needed]}

- The following are equivalent:
- For all

- Every satisfiable set of formulas is consistent, where a set of formulas is satisfiable if and only if there exists a model such that .
- For all and :
- if not , then ;
- if and , then ;
- if , then or .

- Let be a maximally consistent set of formulas and suppose it contains witnesses. For all and :
- if , then ,
- either or ,
- if and only if or ,
- if and , then ,
- if and only if there is a term such that .
^{[citation needed]}

Let be a set of symbols. Let be a maximally consistent set of -formulas containing witnesses.

Define an equivalence relation on the set of -terms by if , where denotes equality. Let denote the equivalence class of terms containing ; and let where is the set of terms based on the set of symbols .

Define the -structure over , also called the **term-structure** corresponding to , by:

- for each -ary relation symbol , define if
^{[8]} - for each -ary function symbol , define
- for each constant symbol , define

Define a variable assignment by for each variable . Let be the **term interpretation** associated with .

Then for each -formula :

if and only if ^{[citation needed]}

There are several things to verify. First, that is in fact an equivalence relation. Then, it needs to be verified that (1), (2), and (3) are well defined. This falls out of the fact that is an equivalence relation and also requires a proof that (1) and (2) are independent of the choice of class representatives. Finally, can be verified by induction on formulas.

In ZFC set theory with classical first-order logic,^{[9]} an **inconsistent** theory is one such that there exists a closed sentence such that contains both and its negation . A **consistent** theory is one such that the following logically equivalent conditions hold

^{[10]}