  In his early papers on the foundations of logic, Popper (1947a; 1947b) gives inferential characterizations of logical constants in proof-theoretic terms. He uses a calculus of sequents and characterizes logical constants by certain derivability conditions of such sequents. His terminology clearly suggests that he intends a proof-theoretic semantics of logical constants, as he speaks of “inferential definitions” and the “trivialization of mathematical logic” achieved by defining constants in the way described. Although his presentation is not free from conceptual imprecision and errors, he was the first to consider the sequent-style inferential behaviour of logical constants to characterize them. This is all the more remarkable as he was probably not at all, and definitely not fully aware of Gentzen’s sequent calculus and Gentzen’s further achievements (he was in correspondence with Bernays, though). However, against his own opinion, his work can better be understood as an attempt to define the logicality of constants and to structurally characterize them, than as a proof-theoretic semantics in the genuine sense. He nevertheless anticipated many ideas now common in proof-theoretic semantics, such as the characterization of logical constants by means of certain minimality or maximality conditions with respect to introduction or elimination rules.