Also called a formal language, and a logistic system. A system in which explicit rules are provided for determining

1. (a) which are the expressions of the system;

2. (b) which sequences of expressions count as well formed (well-formed formulae, or wffs);

3. (c) which sequences of wffs count as proofs.

A system may contain axioms, and a wff that terminates a proof will be a theorem. The two most famous calculi are the propositional calculus and the predicate calculus.