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In mathematics and logic, an operation is **finitary** if it has finite arity, i.e. if it has a finite number of input values. Similarly, an **infinitary** operation is one with an infinite number of input values.

In standard mathematics, an operation is finitary by definition. Therefore, these terms are usually only used in the context of infinitary logic.

A **finitary argument** is one which can be translated into a finite set of symbolic propositions starting from a finite^{[1]} set of axioms. In other words, it is a proof (including all assumptions) that can be written on a large enough sheet of paper.

By contrast, **infinitary logic** studies logics that allow infinitely long statements and proofs. In such a logic, one can regard the existential quantifier, for instance, as derived from an infinitary disjunction.

Logicians in the early 20th century aimed to solve the problem of foundations, such as, "What is the true base of mathematics?" The program was to be able to rewrite all mathematics using an entirely syntactical language *without semantics*. In the words of David Hilbert (referring to geometry), "it does not matter if we call the things *chairs*, *tables* and *beer mugs* or *points*, *lines* and *planes*."

The stress on finiteness came from the idea that human *mathematical* thought is based on a finite number of principles ^{[citation needed]} and all the reasonings follow essentially one rule: the *modus ponens*. The project was to fix a finite number of symbols (essentially the numerals 1, 2, 3, ... the letters of alphabet and some special symbols like "+", "⇒", "(", ")", etc.), give a finite number of propositions expressed in those symbols, which were to be taken as "foundations" (the axioms), and some rules of inference which would model the way humans make conclusions. From these, *regardless of the semantic interpretation of the symbols* the remaining theorems should follow *formally* using only the stated rules (which make mathematics look like a *game with symbols* more than a *science*) without the need to rely on ingenuity. The hope was to prove that from these axioms and rules *all* the theorems of mathematics could be deduced. That aim is known as logicism.

**^**The number of axioms*referenced*in the argument will necessarily be finite since the proof is finite, but the number of axioms from which these are*chosen*is infinite when the system has axiom schemes, e.g. the axiom schemes of propositional calculus.

Look up **finitary** in Wiktionary, the free dictionary.

- Stanford Encyclopedia of Philosophy entry on Infinitary Logic