 Here he seems to overlook that, constructively, there is a difference between the claim that every mathematical problem is solvable and the weaker claim that there are no absolutely unsolvable problems. The former is equivalent to \(A \vee \neg A\), the latter to \(\neg \neg(A \vee \neg A)\); and Brouwer had demonstrated the intuitionistic validity of the latter in the same paper. Indeed, in the Brouwer archive there is a note from about the same period 1907–1908 in which the point is made explicitly: