As an undergraduate, the studies which influenced most powerfully my intellectual development were chemistry, biology, and historical geology. From chemistry I gained an appreciation of exact scientific procedure; biology and geology quickened my imagination with a sense of the vastness and complexity of the march of life. In philosophy I was most influenced by Aristotle's Ethics, Leibniz, Kant, and Green. It was, I think, Green's Prolegomena to Ethics which, more than any other influence, turned me in the direction of philosophy as a vocation-not his metaphysics, but his reverence for the free spirit and his noble passion for social liberty and progress.