Appearing in English for the first time, this book comprises two of Ortega's most important works, ¿Qué es conocimiento? and the essay "Ideas y creencias." This is Ortega's attempt to systematically present the foundations of his metaphysics of human life and, on that basis, to provide a radical philosophical account of knowledge. In so doing, he criticizes idealism and overcomes it. Accordingly, this book goes well beyond a treatise on epistemology; in fact, as understood in modern philosophy, this discipline and its questions are shown to be derivative and, in that sense, they are transcended here by Ortega's systematic effort. Written during the time of his maturity, these works are representative of his fruitful and radical period. Both ¿Qué es conocimiento? and "Ideas y creencias" are equally decisive not only for the understanding and radical completion of Ortega's work, but also for their relevance to the work of continental philosophers during the same period and for years to come (e.g., Husserl, Jaspers, Heidegger, Sartre, and others).