Many contemporary philosophers find intentionality challenging because of its apparent resistance to integration into a naturalistic worldview. Consequently, they strive to make intentionality intelligible as part of the natural world. The central thesis of this book is that such an approach does not do justice to the philosophical significance of the problem. The problem of intentionality lies at a different level, namely the challenge to the principle of the indiscernibility of identicals. Addressing this challenge goes beyond the scope of naturalizing intentionality; instead, it requires logical and semantic considerations to refute it. A promising avenue from this perspective is Frege’s claim that intentional states are relations to propositions. The author elucidates the subtleties of this approach in contrast to alternative positions, defends it against classical counterarguments, and demonstrates its compatibility with directly referentialist views. The objection that speaking of relations to propositions remains unsatisfactory in the absence of any naturalistic comment is refuted after a detailed discussion of Dretske's information-theoretic approach.