Predicate (mathematical logic)

"Predicate (logic)" redirects here. For other uses, see Predicate (disambiguation) § Logic.

In logic, a predicate is a symbol which represents a property or a relation. For instance, in the first order formula $P(a)$, the symbol $P$ is a predicate which applies to the individual constant $a$. Similarly, in the formula $R(a,b)$, $R$ is a predicate which applies to the individual constants $a$ and $b$.

In the semantics of logic, predicates are interpreted as relations. For instance, in a standard semantics for first-order logic, the formula $R(a,b)$ would be true on an interpretation if the entities denoted by $a$ and $b$ stand in the relation denoted by $R$. Since predicates are non-logical symbols, they can denote different relations depending on the interpretation used to interpret them. While first-order logic only includes predicates which apply to individual constants, other logics may allow predicates which apply to other predicates.

Predicates in different systems

• In propositional logic, atomic formulas are sometimes regarded as zero-place predicates In a sense, these are nullary (i.e. 0-arity) predicates.
• In first-order logic, a predicate forms an atomic formula when applied to an appropriate number of terms.
• In set theory with excluded middle, predicates are understood to be characteristic functions or set indicator functions (i.e., functions from a set element to a truth value). Set-builder notation makes use of predicates to define sets.
• In autoepistemic logic, which rejects the law of excluded middle, predicates may be true, false, or simply unknown. In particular, a given collection of facts may be insufficient to determine the truth or falsehood of a predicate.
• In fuzzy logic, predicates are the characteristic functions of a probability distribution. That is, the strict true/false valuation of the predicate is replaced by a quantity interpreted as the degree of truth.

See also

• Classifying topos
• Free variables and bound variables
• Multigrade predicate
• Opaque predicate
• Predicate functor logic
• Predicate variable
• Truthbearer
• Well-formed formula

External links

• Introduction to predicates