Many contemporary philosophers view intentionality as a challenge, as it seems to resist integration into a naturalistic worldview. They therefore strive to make intentionality comprehensible as part of natural phenomena. The central thesis of the book is that this approach does not do justice to the philosophical significance of the topic. The problem of intentionality lies on a completely different level. It consists in the questioning of the principle of the indistinguishability of the identical. This challenge cannot be met through a naturalization of intentionality. Rather, it requires logical-semantic considerations to refute it. The thesis introduced by Frege, that intentional states represent relations to propositions, proves to be promising from this perspective.