 Brouwer’s approach to paradoxes was also based on a contentual conception of mathematics. It can be found in the dissertation of 1907, chapter III (cf. also van Dalen 1999, pp. 105). As to Russell’s contradiction, Brouwer noticed that the usual logical principles only hold for words with mathematical content, not for linguistic systems, like those of Peano or Russell. For instance, in order to decide whether a class falls under a propositional function, the class has to be a completed totality. The contradictions show that there are propositional functions which define complementary (disjoint) classes and yet do not satisfy the tertium non datur. Similar ideas can be found in the philosophical paper of 1908, “On the unreliability of tertium non datur” (see van Dalen 1999).