With the impressive scientific achievements of the seventeenth century and after we are concerned only indirectly. What chiefly interests us at the moment is the connection, in so far as it can be established, between this sudden outburst of scientific achievement and the “climate” of thought in which it took place. Every scientist, even the most practical, makes a number of assumptions into the credentials of which he neglects to enquire, sometimes because he lacks sufficient time to do so, and sometimes because he is unaware that he is assuming anything at all. Often the most important assumptions that he makes are of the latter kind. Thus, when we say that the growth of modern experimental science was due largely to the new philosophical outlook of the time, we must not for a moment suppose that every scientist was consciously striving to prove the practical truth of the new theories. Many scientists never gave the matter a thought. All we can legitimately say is that by concentrating upon the mathematical interpretation of natural phenomena, the scientists made such headway towards understanding certain processes of nature that they were instinctively impelled to neglect those other processes which, in resisting such treatment, they assumed to be outside the scope of science. The result was what Whitehead has called a “closure of nature to mind”: not in the sense that nature represented a closed book to the minds of scientists—on the contrary, they had just begun to open it—but in the45 sense that natural phenomena were held to be governed by laws operating independently of any influence the mind might exert upon them. The mind might, with application, come to discover the nature and working of these laws; it could not influence them or claim any part in their operation. They were characteristics of an “external world” in which mind exerted no authority. 1 The notion that mind could intervene in the orderly working of the natural world would have been considered to involve an inconceivable suspension of the laws of nature; for the intervention of mind implied, for reasons which we have explained, the intervention of an element both irrational and capricious. Needless to say, such a view of nature and mind is still held, or at any rate assumed, by many thinkers even to-day: which is the reason why we ought to examine it with some attention.