Karl Popper on Deduction

We outline Karl Popper’s theory of deduction, which he developed in the 1940s. In his theory it is assumed that a consequence relation is given or otherwise constructed by postulation. Logical operations, which may be available in this consequence relation, are then characterized by means of relational definitions, and logical operators are introduced as names for these operations by means of inferential definitions. Using logically structured sentences thus introduced, the inference laws for them are immediately obtained from the inferential definitions.