Hypotheses such as these find no immediate ground for proof or disproof in the physical world. Let us, therefore, turn to a different field, and, assuming that the human soul is a four-dimensional being, capable in itself of four dimensional movements, but in its experiences through the senses limited to three dimensions, ask if the history of thought, of these productivities which characterise man, correspond to our assumption. Let us pass in review those steps by which man, presumably a four-dimensional being, despite his bodily environment, has come to recognise the fact of four-dimensional existence. Deferring this enquiry to another chapter, I will here recapitulate the argument in order to show that our purpose is entirely practical and independent of any philosophical or metaphysical considerations. If two shots are fired at a target, and the second bullet hits it at a different place to the first, we suppose that there was some difference in the conditions under which the second shot was fired from those affecting the first shot. The force of the powder, the direction of aim, the strength of the wind, or some condition must have been different in the second case, if the course of the bullet was not exactly the same as in the first case. Corresponding to every difference in a result there must be some difference in the antecedent material conditions. By tracing out this chain of relations we explain nature. But there is also another mode of explanation which we apply. If we ask what was the cause that a certain ship was built, or that a certain structure was erected, we might proceed to investigate the changes in the brain cells of the men who designed the works. Every variation in one ship or building from another ship or building is accompanied by a variation in the processes that go on in the brain matter of the designers. But practically this would be a very long task. A more effective mode of explaining the production of the ship or building would be to enquire into the motives, plans, and aims of the men who constructed them. We obtain a cumulative and consistent body of knowledge much more easily and effectively in the latter way. Sometimes we apply the one, sometimes the other mode of explanation. But it must be observed that the method of explanation founded on aim, purpose, volition, always presupposes a mechanical system on which the volition and aim works. The conception of man as willing and acting from motives involves that of a number of uniform processes of nature which he can modify, and of which he can make application. In the mechanical conditions of the three-dimensional world, the only volitional agency which we can demonstrate is the human agency. But when we consider the four-dimensional world the conclusion remains perfectly open.