This paper aims to show that to solve the question of the possibility of scientific progress, Popper had to introduce the idea of truth into his theory of science. This conception of progress, in terms of the notion of truth, was only outlined in the work Conjectures and Refutations (1963). The idea that the goal of science is truth does not appear in his early works. When Popper wrote his The Logic of Scientific Discovery (1934), science was defined in terms of logic-methodological rules, and not of its goals. Scientific progress was conceived based on the notions of testability and corroboration of theories, logical and methodological requirements for a theory to be considered scientific. Popper did not relate the question of scientific progress to notion of truth in this work, because when he wrote it he did not have a consistent theory of truth. It was only after Tarski had written his article on the semantic conception of truth that Popper, based on this conception of truth, could complement his thesis about the progress of science expounded in The Logic of Scientific Discovery with a theory about the content of truth and the approximation to the truth.
scientific progress; truth; testability; corroboration