Psychology has always defined itself as a science and yet it has lacked the theoretical and methodological unity regarded as characteristic of the natural sciences. Nicolò Gaj explores the topical question of unification in psychology, setting out a conceptual framework for considerations of unity and disunity, and exploring the evidence of its fragmentation. He takes a critical look at the history of the most prominent attempts at unification, and at the desirability and feasibility of the whole project. The book represents a unique and valuable attempt to address the issue of unification from a philosophical perspective, and via a combination of theoretical and empirical research.