This is how Geometry, Astronomy, Biology came, in ancient times, to be successively detached from general Philosophy. The separation of Psychology--the detailed study of the processes of mental life--from Philosophy hardly goes back beyond the days of our fathers, and the separation of such studies as 'sociology' from general Philosophy may be said to belong quite definitely to our own time. If our children have leisure for study at all, no doubt they will see the process carried much further. But it is important to bear in mind that neither Philosophy in the narrower sense nor Science in the narrower sense will be fruitfully prosecuted unless the men who are working at each understand that their own labours are only part of a single undivided work. Without a genuine grasp of some department of detailed facts no man is likely to achieve much in the search for principles, for it is by analysis of facts that principles are to be found, and without real insight into broad general principles the worker in detail is likely to achieve nothing but confusion. The antagonism between 'philosophers' and 'men of science' so characteristic of the last half of the nineteenth century has been productive of nothing but evil. It has given us 'philosophers' whose knowledge about the facts with which serious thinking has to deal has been hopelessly inaccurate; it has also given us 'men of science' who have been 'ageometretes' and have, by consequence, when forced to offer some account of first principles, taken refuge in the wildest and weirdest improvisation. For really fruitful work we need the union in one person of the 'man of science' and the 'philosopher', or at least the most intimate co-operation between the two. Our theories of first principles require to be constantly revised, purified, and quickened by contact with knowledge of detailed fact; and our representations of fact call for constant restatement in terms of a system of more and more thoroughly thought-out postulates or first principles. This is perhaps why the department of human knowledge in which the last half-century has seen the most remarkable advances is just that in which unremitting scrutiny of principles has gone most closely hand-in-hand with the mastery of fresh masses of detail, pure mathematics, and again why the present state of what is loosely called 'evolutionary' science is so unsatisfactory to any one who has a high ideal of what a science ought to be. It exhibits at once an enormous mass of detailed information and an apparently hopeless vagueness about the meaning of the 'laws' by which all this detail is to be co-ordinated, the reasons for thinking these laws true, and the precise range of their significance. The work of men like Cantor, Dedekind, Frege, Whitehead, Russell, is providing us with an almost unexceptional theory of the first principles required for pure mathematics. We are already in a position to say with almost complete