I. I propose in this chapter to examine two theses. Though they are two it is convenient to consider both in one paper. For they can be seen best as two opposed, but equally wrong, accounts of the relations, or lack of relations, between two different kinds of explanation which may be offered for various human ongoings. When so described these theses-and hence, of course, their contradictories too-sound like paradigms of philosophy in the strictest sense: and so indeed they are. Yet each-and its contradictory likewise-in fact constitutes also an example of the sort of wide-ranging, exdting, and ideologically relevant contention which professional philosophers nowadays are often supposed to eschew. For if the first was right it would become-to put it no stronger-very difficult to go on maintaining that man is apart of nature; and, surely, this must in some interpretation be a presupposition of anything which deserves the name of a sdentific world-outlook. I If the second was right it would enable us to dispose at one blow, and in a new way, of the problem of recondling physiological determinism with the constantly realized possibility of free, rational and responsible human conduct; and this again would be an achievement of obvious ideological importance. Though we shall be here concerned chiefly with the two theses in themselves, we shall also give some attention to their important corollaries.