The special sciences have all grown up by the use of notions derived from common sense, such as things and their qualities, space, time, and causation. Science itself has shown that none of these common-sense notions will quite serve for the explanation of the world; but it is hardly the province of any special science to undertake the necessary reconstruction of fundamentals. This must be the business of philosophy. I want to say, to begin with, that I believe it to be a business of very great importance. I believe that the philosophical errors in common-sense beliefs not only produce confusion in science, but also do harm in ethics and politics, in social institutions, and in the conduct of everyday life. It will be no part of my business, in this volume, to point out these practical effects of a bad philosophy: my business will be purely intellectual. But if I am right, the intellectual adventures which lie before us have effects in many directions which seem, at first sight, quite remote from our theme. The effect of our passions upon our beliefs forms a favourite subject of modern psychologists; but the converse effect, that of our beliefs upon our passions, also exists, though it is not such as an old-fashioned intellectualist psychology would have supposed. Although I shall not discuss it, we shall do well to bear it in mind, in order to realise that our discussions may have bearings upon matters lying outside the sphere of pure intellect. I mentioned a moment ago three defects in common beliefs, namely, that they are cocksure, vague, and self-contradictory. It is the business of philosophy to correct these defects so far as it can, without throwing over knowledge altogether. To be a good philosopher, a man must have a strong desire to know, combined with great caution in believing that he knows; he must also have logical acumen and the habit of exact thinking. All these, of course, are a matter of degree. Vagueness, in particular, belongs, in some degree, to all human thinking; we can diminish it indefinitely, but we can never abolish it wholly. Philosophy, accordingly, is a continuing activity, not something in which we can achieve final perfection once for all. In this respect, philosophy has suffered from its association with theology. Theological dogmas are fixed, and are regarded by the orthodox as incapable of improvement. Philosophers have too often tried to produce similarly final systems: they have not been content with the gradual approximations that satisfied men of science. In this they seem to me to have been mistaken. Philosophy should be piecemeal and provisional like science; final truth belongs to heaven, not to this world. The three defects which I have mentioned are interconnected, and by becoming aware of any one we may be led to recognise the other two. I will illustrate all three by a few examples.