Few things are more firmly established in popular philosophy than the distinction between mind and matter. Those who are not professional metaphysicians are willing to confess that they do not know what mind actually is, or how matter is constituted; but they remain convinced that there is an impassable gulf between the two, and that both belong to what actually exists in the world. Philosophers, on the other hand, have maintained often that matter is a mere fiction imagined by mind, and sometimes that mind is a mere property of a certain kind of matter. Those who maintain that mind is the reality and matter an evil dream are called "idealists"--a word which has a different meaning in philosophy from that which it bears in ordinary life. Those who argue that matter is the reality and mind a mere property of protoplasm are called "materialists." They have been rare among philosophers, but common, at certain periods, among men of science. Idealists, materialists, and ordinary mortals have been in agreement on one point: that they knew sufficiently what they meant by the words "mind" and "matter" to be able to conduct their debate intelligently. Yet it was just in this point, as to which they were at one, that they seem to me to have been all alike in error. The stuff of which the world of our experience is composed is, in my belief, neither mind nor matter, but something more primitive than either. Both mind and matter seem to be composite, and the stuff of which they are compounded lies in a sense between the two, in a sense above them both, like a common ancestor. As regards matter, I have set forth my reasons for this view on former occasions,* and I shall not now repeat them. But the question of mind is more difficult, and it is this question that I propose to discuss in these lectures. A great deal of what I shall have to say is not original; indeed, much recent work, in various fields, has tended to show the necessity of such theories as those which I shall be advocating. Accordingly in this first lecture I shall try to give a brief description of the systems of ideas within which our investigation is to be carried on. * "Our Knowledge of the External World" (Allen & Unwin), Chapters III and IV. Also "Mysticism and Logic," Essays VII and VIII.