First published in 2002. This is Volume XV of seventeen in the Philosophy of Mind and Psychology series. The Muirhead Library of Philosophy 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, Aesthetics, Political Philosophy, Theology. Written in 1976, this is a collection of essays by a number of well-known philosophers who were invited to write on whichever philosophical issue relating to psychical research interested them most.