This book examines the subject-matter of psychology against the background of the philosophical problem of reality. It locates the core of the issue in the dualistic conception of reality which, it argues, has left psychologists with a conceptually constrained choice of subject-matter, for instance between experience and behavior, not to mention philosophically incomplete ways of discussing its possible subject-matters. By drawing on the work of philosophers and philosophically informed psychologists, the book seeks to explain and advance the long-standing debate by introducing the under-utilized perspective of ideal-realism. The historical origins and trajectory of ideal-realism are recovered from the late 18th century through the early 20th century. The exposition of this tradition, especially in the little-known German Realpsychologie, provides theoretical psychology with a philosophically founded understanding of its subject-matter.