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PHILOSOPHY AND THE STUDY OF PHILOSOPHERS. 3 its axis. So again, while the Darwinian theory of natural selection has incidentally led to the observation of many previously unnoticed facts about plants and animals, the theory itself is not an addition to our sum of facts but the substitution of a new definition of" species " for an old one. Indeed the old type of naturalist often collected many more facts (and specimens) than the new biologist, but his facts lay simply alongside of one another and were not held together by a continuous thread of theory. Now, if the sciences, being, in Mr. Herbert Spencer's phrase, " partially unified knowledge," have their history marked by changes in their unifying concepts, much more must this be the case with philosophy, which attempts to reach complete unification of knowledge I say " attempts to reach," for, just as Pythagoras took "lover of wisdom " and not " wise " for his title, so must we define philosophy, not as " completely unified knowledge," but as the effort or endeavour to reach it. A new fact may upset an accepted scientific theory ; but not merely every new fact and every new event in the physical universe, but every new scientific theory (being a new event in the region of mind) is a new fact for philosophy, and the unifying and systematising con- cepts of philosophy must therefore be constantly subject to revision and modification. The main occupation of philo- sophy comes therefore to be the examination of those con- cepts that serve well enough for the provisional and very partial unifications of ordinary life, and of those concepts that serve well enough for the unification of the special sciences which deal with particular aspects of the world taken in abstraction from one another. Philosophy must primarily and at least be " a criticism of categories ". The criticism consists in seeking to discover the relations of the various concepts, which are used uncritically in ordinary and in scientific thinking, to the ultimate data of experience (and what these are it is of course the concern of philosophy to discover, for common-sense and even the special sciences are alike content to accept as facts what are really un analysed and unconscious theories) ; but beyond this primary task of philosophical criticism, it must, in the very effort to be thoroughgoing, proceed to consider the relation of these fundamental concepts to one another, and in doing this even a professedly critical philosophy must to some extent become constructive and speculative. If we define philosophy (or metaphysics) as the theory of reality and its method as the analysis of experience, I do not think this can bring us to a conception of philosophy and of