logistic system a formal language together with a set of axioms and rules of inference, or what many today would call a ‘logic.’ The original idea behind the notion of a logistic system was that the language, axioms, rules, and attendant concepts of proof and theorem were to be specified in a mathematically precise fashion, thus enabling one to make the study of deductive reasoning an exact science. One was to begin with an effective specification of the primitive symbols of the language and of which (finite) sequences of symbols were to count as sentences or wellformed formulas. Next, certain sentences were to be singled out effectively as axioms. The rules of inference were also to be given in such a manner that there would be an effective procedure for telling which rules are rules of the system and what inferences they license. A proof was then defined as any finite sequence of sentences, each of which is either an axiom or follows from some earlier line(s) by one of the rules, with a theorem being the last line of a proof. With the subsequent development of logic, the requirement of effectiveness has sometimes been dropped, as has the requirement that sentences and proofs be finite in length. See also ALGORITHM , INFINI- TARY LOGIC , PROOF THEOR. G.F.S.