Karl R. Popper (1947a, b) took the central topic of logic to be the theory of deductive inference and the main problem he was concerned with in his early writings on deductive logic was to give a satisfactory definition for the notion of "deductive valid inference". Although his definition was supposed to be a generalization of Tarski's definition of logical consequence, Popper showed that the notion of "truth" can be avoided, even though its use is not objectionable. In addition, unlike Tarski's modeltheoretic (or, better, group-theoretic) criterion, he proposed an inferential criterion to draw a line between the formative (i.e., logical) and the non-formative signs (i.e., non-logical) and defined the validity of deductive inferences on the basis of inferential definitions. 1 Although the notion of truth can be avoided in this inferential foundational approach to deductive inferences, in some latter writings Popper acknowledged that the validity of deduction goes beyond the signs and the rules that govern their use, and emphasized the constitutive role that the notion of truth has for logical deduction: Deduction, I contend, is not valid because we choose or decide to adopt its rules as a standard, or decree that they shall be accepted; rather, it is valid because it adopts, and incorporates, the rules by which truth is transmitted from (logically stronger) premises to (logically weaker) conclusions, and by which falsity is re-transmitted from conclusions to premises. (This retransmission of falsity makes formal logic the Organon of rational criticism -that is, of refutation). (Popper 1962, 64) In addition, Popper et al. (1970, 18) took the notion of truth to be crucial for the applications of logic in the other areas of inquiry. With regard to these applications,