That our thinking, and with it our feeling, is largely conditioned by assumptions which have no logical necessity, is a commonplace of philosophy, and is indeed apparent to the slightest introspection. Characteristic of any age is a body of beliefs, resting on more or less good evidence, and a group of feelings associated with those beliefs. The German language, so rich in indefinite but valuable general terms, afforded the word _Zeitgeist_ for this complex, a word we have directly translated into the Spirit of the Age. The name is a good one; it indicates that we are dealing with something which is widely diffused and also subject to change. It is subject to change, but it plays a dominating rôle in the age to which it belongs. The Spirit of the Age is something that practically all the intellectual life of the age has in common. It is not manifested only in philosophical treatises or in works of art; it is often manifested even more strikingly in statesmen’s speeches and a country’s domestic and foreign policy. It is a kind of intellectual and emotional atmosphere of which everybody is aware, but which probably nobody could define. We see, however, that a very important part of it consists of a sense of probability, of a tendency to accept certain kinds of explanation and to reject others. For the last few decades, at any rate, Science has been the chief factor in forming this omnipresent sense of probability. As a matter of fact, it is probable that the influence of Science in forming the Spirit of the Age can be traced a very long way back, as far back as Copernicus. Not that we assert the existence of a close connection between the Science and the other intellectual activities of Copernicus’s own age. The influence of which we speak is likely to manifest itself gradually; in particular, it may take a long time to affect the arts. And by the time it has percolated so far its origin may be forgotten; it may appear as a subconscious rather than as a conscious group of assumptions. By the time a scientific discovery becomes part of the mental furniture of an age, many of what were originally its possible implications will have become an integral part of it. The original discovery will then be merely the nucleus of a rich intellectual and, possibly, emotional complex, of which the parts are no longer envisaged separately. The work of Newton, for example, and the great body of exact investigations he made possible, influenced the outlook of the nineteenth century chiefly in the direction of making determinism plausible. Such lecturers as Tyndall could confidently appeal to this mental predisposition on the part of their audience, although they had no need to postulate any direct acquaintance with the work of Newton and of his successors. The fact that Newton successfully formulated exact laws for the description of natural phenomena is the important aspect of his work from our present point of view. The