This book introduces the profound reflections of Xavier Zubiri (1898-1983) on the history of philosophy to English-speaking audiences. As a philosopher who rethought much of philosophy and theology, Zubiri felt it necessary to be in continuous dialogue with earlier thinkers both to avoid past mistakes and to extract all that is valuable from them. The theme of the present book is the transcendental in Western philosophy and how a firm grasp of it reveals underlying unity in Western philosophy, but also fundamental problems that Zubiri believed require a complete rethinking of certain basic notions and theories. Zubiri develops this theme by analyzing the work of six major philosophers: Aristotle, St. Thomas, Descartes, Leibniz, Kant, and Hegel. To conclude, he sketches his own resolution of the problems of Western philosophy, a subject addressed in greater depth in his major work, Sentient Intelligence. This translation was made possible by a grant from the Spanish Ministry of Culture.