This is Volume X of seventeen in a collection of works on the Philosophy of Mind and Psychology in the Library of Philosophy which was designed as a contribution to the History of Modern Philosophy under the heads: first of Different Schools of Thought-Sensationalist, Realist, Idealist, Intuitivist; secondly of different Subjects-Psychology, Ethics, Political Philosophy, Theology. Originally published in 1970, this volume brings together essays on Hypothesis and Perception.