This paper aims at clarifying the nature of Frege's system of logic, as presented in the first volume of the Grundgesetze . We undertake a rational reconstruction of this system, by distinguishing its propositional and predicate fragments. This allows us to emphasise the differences and similarities between this system and a modern system of classical second-order logic.
Frege; Second-Order Logic; Grundgesetze der Arithmetik; Begriffsschrift; Frege's notion of a Function.