The events of the physical order, it may be urged, cannot be expressions of the more or less conscious purposes and interests of individual centres of experience, and that for a simple reason. How a purposive agent will behave is always a mystery, except to those who actually understand his purposes. It is impossible, apart from actual insight into those purposes, to infer from the mere examination of his past behaviour what his behaviour in the future will be. For the special characteristic of purposive action is its power to find new ways of response to stimulus. Hence it is that we rightly regard the power to learn by experience, that is, to acquire more and more _appropriate_ reactions to stimulus, as the test of a creature’s intelligence. Where there is no progressive adaptability there is no ground to assume intelligence and purpose. Hence again the impossibility of calculating beforehand with any certainty what course the behaviour of an intelligent being will take, unless you are actually aware of the purposes he is seeking to realise. Now, except in the case of the organic world, it may be urged, we do not find progressive _adaptability_ in Nature. The inorganic constituents of the physical order always react with absolute uniformity in the same way upon the same environment. Their behaviour exhibits absolutely undeviating conformity to general routine laws of sequence, and can therefore be calculated beforehand, provided that the resources of our mathematics are adequate to deal with the problems it presents, with absolute exactitude and certainty. That this routine uniformity exists in physical nature is, in fact, a fundamental principle in the logic of inductive science. Every indication of sentience and purpose is thus absent from physical nature, outside the world of living organisms; it is a realm of rigid conformity to laws of sequence. And these sequences, because absolutely without exception and incapable of modification, are purely _mechanical_, _i.e._ non-purposive and non-intelligent. Nature is, in fact, a complicated _mechanism_, in which every event follows from its conditions with undeviating _necessity_. Views of this kind are often supposed to be logically necessitated by the principles of physical science. It is manifest that if they are sound our whole preceding interpretation of the physical order is invalidated. For this reason, as well as because of the far-reaching consequences often drawn from them as to human freedom and moral responsibility, it will be necessary to examine their foundation in some detail.