Strict implication, deducibility and the deduction theorem

Abstract

Lewis and Langford state, “… it appears that the relation of strict implication expresses precisely that relation which holds when valid deduction is possible. It fails to hold when valid deduction is not possible. In that sense, the system of strict implication may be said to provide that canon and critique of deductive inference which is the desideratum of logical investigation.” Neglecting for the present other possible criticisms of this assertion, it is plausible to maintain that if strict implication is intended to systematize the familiar concept of deducibility or entailment, then some form of the deduction theorem should hold for it. The purpose of this paper is to analyze and extend some results previously established which bear on the problem.We will begin with a rough statement of some relevent considerations. Let the system S contain among its connectives an implication connective ‘I’ and a conjunction connective ‘&’. Let A₁, A₂, …, A_n ⊦ B abbreviate that B is provable on the hypotheses A₁, A₂, …, A_n for a suitable definition of “proof on hypotheses”, where A₁, A₂, …, A_n, B are well-formed expressions of S.