On the semantic non-completeness of certain Lewis calculi

Abstract

I intend to show that for S1, S2, and S3 the class of true formulas cannot coincide with the class of theorems. This is to hold irrespective of the meaning assigned to “◊”. when only “∼”, “▪”, and the variables are interpreted in the customary manner.The idea of the proof is extremely simple and can be illustrated by the following argument concerning S3. The formula ~(◊(p▪~p)⥽▪p▪~p) ∨ (◊(q▪~q)⥽▪q▪~q) is a theorem of S3. Suppose now that all S3-theorems are true. Then either ~(◊(p▪~p)⥽▪p▪~p) is true (for every p) or (◊(q▪~q)⥽▪q▪~q) is true (for every q). But none of these formulas is an S3-theorem. Then some true S3-formula is not an S3-theorem. Hence, if all S3-theorems are true, some true S3-formula is not an S3-theorem. Then the class of S3-theorems cannot be identical with the class of true S3-formulas.