# Higher-order logic

In mathematics and logic, a higher-order logic is a form of predicate logic that is distinguished from first-order logic by additional quantifiers and a stronger semantics. Higher-order logics with their standard semantics are more expressive, but their model-theoretic properties are less well-behaved than those of first-order logic.

## Higher-order simple predicate logic

The term "higher-order logic", abbreviated as HOL, is commonly used to mean higher order simple predicate logic. Here "simple" indicates that the underlying type theory is simple, not polymorphic or dependent.[1]

There are two possible semantics for HOL. In the standard or full semantics, quantifiers over higher-type objects range over all possible objects of that type. For example, a quantifier over sets of individuals ranges over the entire powerset of the set of individuals. Thus, in standard semantics, once the set of individuals is specified, this is enough to specify all the quantifiers.

HOL with standard semantics is more expressive than first-order logic. For example, HOL admits categorical axiomatizations of the natural numbers, and of the real numbers, which are impossible with first-order logic. However, by a result of Gödel, HOL with standard semantics does not admit an effective, sound and complete proof calculus.

The model-theoretic properties of HOL with standard semantics are also more complex than those of first-order logic. For example, the Löwenheim number of second-order logic is already larger than the first measurable cardinal, if such a cardinal exists.[2] The Löwenheim number of first-order logic, in contrast, is 0, the smallest infinite cardinal.

In Henkin semantics, a separate domain is included in each interpretation for each higher-order type. Thus, for example, quantifiers over sets of individuals may range over only a subset of the powerset of the set of individuals. HOL with these semantics is equivalent to many-sorted first-order logic, rather than being stronger than first-order logic. In particular, HOL with Henkin semantics has all the model-theoretic properties of first-order logic, and has a complete, sound, effective proof system inherited from first-order logic.

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## Examples

Examples of higher order logics include HOL, Church's Simple Theory of Types, Thierry Coquand's calculus of constructions, which allows for both dependent and polymorphic types.

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## Criticism

Quine has criticized higher-order logic (with standard semantics) as "set theory in sheep's clothing". Quine's criticism focuses on the lack of an effective, sound, complete proof theory; he argues that this makes HOL not a "logic". Shapiro has responded to this criticism, arguing that the additional semantic expressiveness can offset the lack of a proof theory, and arguing that a "logic" need only have a deductive system or a semantical system, but perhaps may not have both.

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## References

1. ^ Jacobs, 1999, chapter 5
2. ^ Menachem Magidor and Jouko Väänänen. "On Löwenheim-Skolem-Tarski numbers for extensions of first order logic", Report No. 15 (2009/2010) of the Mittag-Leffler Institute.
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