### Consistent

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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 that a theory is consistent if and only if there is no formula *P* such that both *P* and its negation are provable from the axioms of the theory under its associated deductive system.

If these semantic and syntactic definitions are equivalent for a particular deductive logic, the logic is **complete**.** The completeness of the sentential calculus was proved by Paul Bernays in 1918**^{[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. 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.

## Contents

## Consistency and completeness in arithmetic and set theory

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 effective 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 effective 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, effective, consistent theory of arithmetic can never be proven in that system itself. The same result is true for effective theories that can describe a strong enough fragment of arithmetic – including set theories such as Zermelo–Fraenkel set theory. These set theories cannot prove their own Gödel sentences – 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*.

## First-order logic

### Notation

⊢ (Turnstyle symbol) in the following context of Mathematical logic, means "provable from". That is, a ⊢ b reads: b is provable from a (in some specified formal system) -- see List of logic symbols) . In other cases, the turnstyle symbol may stand to mean infers; derived from. See: List of mathematical symbols.

### Definition

A set of formulas $\backslash Phi$ in first-order logic is **consistent** (written Con$\backslash Phi$) if and only if there is no formula $\backslash phi$ such that $\backslash Phi\; \backslash vdash\; \backslash phi$ and $\backslash Phi\; \backslash vdash\; \backslash lnot\backslash phi$. Otherwise $\backslash Phi$ is **inconsistent** and is written Inc$\backslash Phi$.

$\backslash Phi$ is said to be **simply consistent** if and only if for no formula $\backslash phi$ of $\backslash Phi$, both $\backslash phi$ and the negation of $\backslash phi$ are theorems of $\backslash Phi$.

$\backslash Phi$ is said to be **absolutely consistent** or **Post consistent** if and only if at least one formula of $\backslash Phi$ is not a theorem of $\backslash Phi$.

$\backslash Phi$ is said to be **maximally consistent** if and only if for every formula $\backslash phi$, if Con ($\backslash Phi\; \backslash cup\; \backslash phi$) then $\backslash phi\; \backslash in\; \backslash Phi$.

$\backslash Phi$ is said to **contain witnesses** if and only if for every formula of the form $\backslash exists\; x\; \backslash phi$ there exists a term $t$ such that $(\backslash exists\; x\; \backslash phi\; \backslash to\; \backslash phi\; \{t\; \backslash over\; x\})\; \backslash in\; \backslash Phi$. See First-order logic.

### Basic results

- The following are equivalent:
- Inc$\backslash Phi$
- For all $\backslash phi,\backslash ;\; \backslash Phi\; \backslash vdash\; \backslash phi.$

- Every satisfiable set of formulas is consistent, where a set of formulas $\backslash Phi$ is satisfiable if and only if there exists a model $\backslash mathfrak\{I\}$ such that $\backslash mathfrak\{I\}\; \backslash vDash\; \backslash Phi$.
- For all $\backslash Phi$ and $\backslash phi$:
- if not $\backslash Phi\; \backslash vdash\; \backslash phi$, then Con$\backslash left(\; \backslash Phi\; \backslash cup\; \backslash \{\backslash lnot\backslash phi\backslash \}\backslash right)$;
- if Con $\backslash Phi$ and $\backslash Phi\; \backslash vdash\; \backslash phi$, then Con$\backslash left(\backslash Phi\; \backslash cup\; \backslash \{\backslash phi\backslash \}\backslash right)$;
- if Con $\backslash Phi$, then Con$\backslash left(\; \backslash Phi\; \backslash cup\; \backslash \{\backslash phi\backslash \}\backslash right)$ or Con$\backslash left(\; \backslash Phi\; \backslash cup\; \backslash \{\backslash lnot\; \backslash phi\backslash \}\backslash right)$.

- Let $\backslash Phi$ be a maximally consistent set of formulas and contain witnesses. For all $\backslash phi$ and $\backslash psi$:
- if $\backslash Phi\; \backslash vdash\; \backslash phi$, then $\backslash phi\; \backslash in\; \backslash Phi$,
- either $\backslash phi\; \backslash in\; \backslash Phi$ or $\backslash lnot\; \backslash phi\; \backslash in\; \backslash Phi$,
- $(\backslash phi\; \backslash or\; \backslash psi)\; \backslash in\; \backslash Phi$ if and only if $\backslash phi\; \backslash in\; \backslash Phi$ or $\backslash psi\; \backslash in\; \backslash Phi$,
- if $(\backslash phi\backslash to\backslash psi)\; \backslash in\; \backslash Phi$ and $\backslash phi\; \backslash in\; \backslash Phi$, then $\backslash psi\; \backslash in\; \backslash Phi$,
- $\backslash exists\; x\; \backslash phi\; \backslash in\; \backslash Phi$ if and only if there is a term $t$ such that $\backslash phi\{t\; \backslash over\; x\}\backslash in\backslash Phi$.

### Henkin's theorem

Let $\backslash Phi$ be a maximally consistent set of $S$-formulas containing witnesses.

Define a binary relation $\backslash sim$ on the set of $S$-terms such that $t\_0\; \backslash sim\; t\_1$ if and only if $\backslash ;\; t\_0\; \backslash equiv\; t\_1\; \backslash in\; \backslash Phi$; and let $\backslash overline\; t\; \backslash !$ denote the equivalence class of terms containing $t\; \backslash !$; and let $T\_\{\backslash Phi\}:=\; \backslash \{\; \backslash ;\; \backslash overline\; t\; \backslash ;\; |\backslash ;\; t\; \backslash in\; T^S\; \backslash \}$ where $T^S\; \backslash !$ is the set of terms based on the symbol set $S\; \backslash !$.

Define the $S$-structure $\backslash mathfrak\; T\_\{\backslash Phi\}$ over $T\_\{\backslash Phi\}\; \backslash !$ the **term-structure** corresponding to $\backslash Phi$ by:

- for $n$-ary $R\; \backslash in\; S$, $R^\{\backslash mathfrak\; T\_\{\backslash Phi\}\}\; \backslash overline\; \{t\_0\}\; \backslash ldots\; \backslash overline\; \{t\_\{n-1\}\}$ if and only if $\backslash ;\; R\; t\_0\; \backslash ldots\; t\_\{n-1\}\; \backslash in\; \backslash Phi$;
- for $n$-ary $f\; \backslash in\; S$, $f^\{\backslash mathfrak\; T\_\{\backslash Phi\}\}\; (\backslash overline\; \{t\_0\}\; \backslash ldots\; \backslash overline\; \{t\_\{n-1\}\}):=\; \backslash overline\; \{f\; t\_0\; \backslash ldots\; t\_\{n-1\}\}$;
- for $c\; \backslash in\; S$, $c^\{\backslash mathfrak\; T\_\{\backslash Phi\}\}:=\; \backslash overline\; c$.

Let $\backslash mathfrak\; I\_\{\backslash Phi\}:=\; (\backslash mathfrak\; T\_\{\backslash Phi\},\backslash beta\_\{\backslash Phi\})$ be the **term interpretation** associated with $\backslash Phi$, where $\backslash beta\; \_\{\backslash Phi\}\; (x):=\; \backslash bar\; x$.

For all $\backslash phi$, $\backslash ;\; \backslash mathfrak\; I\_\{\backslash Phi\}\; \backslash vDash\; \backslash phi$ if and only if $\backslash ;\; \backslash phi\; \backslash in\; \backslash Phi$.

### Sketch of proof

There are several things to verify. First, that $\backslash sim$ is an equivalence relation. Then, it needs to be verified that (1), (2), and (3) are well defined. This falls out of the fact that $\backslash sim$ is an equivalence relation and also requires a proof that (1) and (2) are independent of the choice of $t\_0,\; \backslash ldots\; ,t\_\{n-1\}$ class representatives. Finally, $\backslash mathfrak\; I\_\{\backslash Phi\}\; \backslash vDash\; \backslash Phi$ can be verified by induction on formulas.

## See also

**
**

has a collection of quotations related to: Consistency |

- Equiconsistency
- Hilbert's problems
- Hilbert's second problem
- Jan Łukasiewicz
- Paraconsistent logic
- ω-consistency

## Footnotes

## References

- Stephen Kleene, 1952 10th impression 1991,
*Introduction to Metamathematics*, North-Holland Publishing Company, Amsterday, New York, ISBN 0-7204-2103-9. - Hans Reichenbach, 1947,
*Elements of Symbolic Logic*, Dover Publications, Inc. New York, ISBN 0-486-24004-5, - Alfred Tarski, 1946,
*Introduction to Logic and to the Methodology of Deductive Sciences, Second Edition*, Dover Publications, Inc., New York, ISBN 0-486-28462-X. - Jean van Heijenoort, 1967,
*From Frege to Gödel: A Source Book in Mathematical Logic*, Harvard University Press, Cambridge, MA, ISBN 0-674-32449-8 (pbk.) - The Cambridge Dictionary of Philosophy,
*consistency* - H.D. Ebbinghaus, J. Flum, W. Thomas,
**Mathematical Logic** - Jevons, W.S., 1870,
*Elementary Lessons in Logic*

## External links

- Chris Mortensen, Stanford Encyclopedia of Philosophy