# Consistency

A Consistency is a statement that does contain a contradiction.

**See:**Contradiction, Curry-Howard, Classical Logic, Deductive Logic, Logic, Theory (Mathematical Logic), Paraconsistent Logic, if And Only if, Model Theory#First-Order Logic, Interpretation (Logic), Well-Formed Formula, Term Logic.

## References

### 2017

- (Wikipedia, 2017) ⇒ https://en.wikipedia.org/wiki/Consistency Retrieved:2017-6-4.
- In classical deductive logic, a
**consistent**theory is one that does not contain a contradiction.^{[1]}^{[2]}The lack of contradiction can be defined in either semantic or syntactic terms. The semantic definition states that a theory is consistent if and only 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 [math]\displaystyle{ T }[/math] is consistent if and only if there is no formula [math]\displaystyle{ \phi }[/math] such that both [math]\displaystyle{ \phi }[/math] and its negation [math]\displaystyle{ \lnot\phi }[/math] are elements of the set [math]\displaystyle{ T }[/math] . Let [math]\displaystyle{ A }[/math] be set of closed sentences (informally "axioms") and [math]\displaystyle{ \langle A\rangle }[/math] the set of closed sentences provable from [math]\displaystyle{ A }[/math] under some (specified, possibly implicitly) formal deductive system. The set of axioms [math]\displaystyle{ A }[/math] is**consistent**when [math]\displaystyle{ \langle A \rangle }[/math] is.^{[3]}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**. The completeness of the sentential calculus was proved by Paul Bernays in 1918^{[4]}and Emil Post in 1921,^{[5]}while the completeness of predicate calculus was proved by Kurt Gödel in 1930,^{[6]}and consistency proofs for arithmetics restricted with respect to the induction axiom schema were proved by Ackermann (1924), von Neumann (1927) and Herbrand (1931).^{[7]}Stronger logics, such as second-order logic, are not complete. A**consistency proof**is a mathematical proof that a particular theory is consistent.^{[8]}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 incompleteness theorems, which showed that sufficiently strong proof theories cannot prove their own consistency (provided that they are in fact consistent).Although consistency can be proved by means of 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 obviously no cut-free proof of falsity, there is no contradiction in general.

- In classical deductive logic, a

- ↑ Tarski 1946 states it this way: "A deductive theory is called CONSISTENT or NON-CONTRADICTORY if no two asserted statements of this theory contradict each other, or in other words, if of any two contradictory sentences . . . at least one cannot be proved," (p. 135) where Tarski defines
*contradictory*as follows: "With the help of the word*not*one forms the NEGATION of any sentence; two sentences, of which the first is a negation of the second, are called CONTRADICTORY SENTENCES" (p. 20). This definition requires a notion of "proof". Gödel in his 1931 defines the notion this way: "The class of*provable formulas*is defined to be the smallest class of formulas that contains the axioms and is closed under the relation "immediate consequence", i.e., formula*c*of*a*and*b*is defined as an*immediate consequence*in terms of*modus ponens*or substitution; cf Gödel 1931 van Heijenoort 1967:601. Tarski defines "proof" informally as "statements follow one another in a definite order according to certain principles . . . and accompanied by considerations intended to establish their validity[true conclusion for all true premises -- Reichenbach 1947:68]" cf Tarski 1946:3. Kleene 1952 defines the notion with respect to either an induction or as to paraphrase) a finite sequence of formulas such that each formula in the sequence is either an axiom or an "immediate consequence" of the preceding formulas; "A*proof is said to be a proof*of*its last formula, and this formula is said to be*(formally) provable*or be a*(formal) theorem" cf Kleene 1952:83. - ↑ see Paraconsistent logic
- ↑
Let [math]\displaystyle{ L }[/math] be a signature, [math]\displaystyle{ T }[/math] a theory in [math]\displaystyle{ L_{\infty \omega} }[/math] and [math]\displaystyle{ \phi }[/math] a sentence in [math]\displaystyle{ L_{\infty\omega} }[/math] . We say that [math]\displaystyle{ \phi }[/math] is a

**consequence**of [math]\displaystyle{ T }[/math] , or that [math]\displaystyle{ T }[/math]**entails**[math]\displaystyle{ \phi }[/math] , in symbols [math]\displaystyle{ T \vdash \phi }[/math] , if every model of [math]\displaystyle{ T }[/math] is a model of [math]\displaystyle{ \phi }[/math] . (In particular if [math]\displaystyle{ T }[/math] has no models then [math]\displaystyle{ T }[/math] entails [math]\displaystyle{ \phi }[/math] .)**Warning**: we don't require that if [math]\displaystyle{ T \vdash \phi }[/math] then there is a proof of [math]\displaystyle{ \phi }[/math] from [math]\displaystyle{ T }[/math] . In any case, with infinitary languages it's not always clear what would constitute a proof. Some writers use [math]\displaystyle{ T\vdash\phi }[/math] to mean that [math]\displaystyle{ \phi }[/math] is deducible from [math]\displaystyle{ T }[/math] in some particular formal proof calculus, and they write [math]\displaystyle{ T \models \phi }[/math] for our notion of entailment (a notation which clashes with our [math]\displaystyle{ A \models \phi }[/math]). For first-order logic the two kinds of entailment coincide by the completeness theorem for the proof calculus in question. We say that [math]\displaystyle{ \phi }[/math] is**valid**, or is a**logical theorem**, in symbols [math]\displaystyle{ \vdash \phi }[/math] , if [math]\displaystyle{ \phi }[/math] is true in every [math]\displaystyle{ L }[/math] -structure. We say that [math]\displaystyle{ \phi }[/math] is**consistent**if [math]\displaystyle{ \phi }[/math] is true in some [math]\displaystyle{ L }[/math] -structure. Likewise we say that a theory [math]\displaystyle{ T }[/math] is**consistent**if it has a model. We say that two theories S and T in L infinity omega are equivalent if they have the same models, i.e. if Mod(S) = Mod(T). (Please note definition of Mod(T) on p. 30 ...)*A Shorter Model Theory*by Wilfrid Hodges, p. 37 - ↑ van Heijenoort 1967:265 states that Bernays determined the
*independence*of the axioms of*Principia Mathematica*, a result not published until 1926, but he says nothing about Bernays proving their*consistency*. - ↑ Post proves both consistency and completeness of the propositional calculus of PM, cf van Heijenoort's commentary and Post's 1931
*Introduction to a general theory of elementary propositions*in van Heijenoort 1967:264ff. Also Tarski 1946:134ff. - ↑ cf van Heijenoort's commentary and Gödel's 1930
*The completeness of the axioms of the functional calculus of logic*in van Heijenoort 1967:582ff - ↑ cf van Heijenoort's commentary and Herbrand's 1930
*On the consistency of arithmetic*in van Heijenoort 1967:618ff. - ↑ Informally, Zermelo–Fraenkel set theory is ordinarily assumed; some dialects of informal mathematics customarily assume the axiom of choice in addition.