Etienne Gilson was one of the most influential intellectuals and philosophers of the twentieth century. Some have credited him with expanding the spectrum of philosophical thought that had previously been limited by nineteenth-century analysts and positivists. Gilson devoted six decades to the study of the major philosophical figures of the Middle Ages. His interpretations of them are justly seen as new and insightful, and have exercised enormous influence on research in philosophy and on its presentation in the classroom. A ‘Gilson Society’ has been active for years, and the Institut catholique in Paris has created a Gilson Chair in Metaphysics. A French publisher has announced a multi-volume publication of his complete works. These seven previously unpublished lectures - Gilson termed them ‘Quests’ - represent his mature thought on three key philosophical questions: the nature of philosophy, ‘species, ’ and ‘matter.’ These are issues of perennial and pertinent interest to both philosophers and scientists. Gilson presents them here with his characteristic clarity, sense, and humour