In the forties, Carnap devised a definition for the concept degree of confirmation for a class of simple languages, hoping to extend it to more comprehensive languages later. This definition was the basis of his project of inductive logic, which he conceived as a branch of semantics. In this paper, I will give an overview of: i) Carnap's first attempt at a definition of degree of confirmation; ii) some objections this attempt had to face; iii) Carnap's amendments, which led to other theories of confirmation.