The explanation of change or movement has always been a central concern of philosophers. Some, like Aristotle, have taken the movement of living things as their paradigm, and tried to explain all movement or change in that way. Others, after the fashion of Descartes, concentrate on the movement of inanimate things and generalise explanations of this to encompass all movement or change. For Aristotle, things have a principle of growth, organisation and movement in their own right. The movement or change of a natural thing is explained by its tendency to move in that way. The line he draws is not, as the line which we would perhaps like to draw is, between organic and inorganic things, but between these grouped together as subject to the same kind of explanation and, on the other hand, artificial things. A problem that results from this division is that while it might seem plausible to explain changes which occur in a baby when it grows into a man by saying that babies naturally tend to grow into men, and if they do not then something has interfered with their natural development, it seems odd to treat inorganic things in this way. Restricted to the contrast between the natural and the artificial, the explanation of stones falling when unsupported is clearly going to provide some difficulty. Although it is true that Aristotle does not think that because in the case of man the form with which matter is formed to make that substance is called a soul, that therefore any kind of form joined with matter to make a substance is called a soul, nevertheless the explanation of things which are a combination, a natural combination, of form and matter is the same for both man and other substances. Confronted with the explanation of falling bodies in this way, it first of all seems implausible and then suggests that things should happen which in fact do not. If it is assumed that the principle of movement is in the stone, ought it not to be assumed that the principle of stopping is in it too? Babies grow into men because it is in their nature to do so perhaps, but that stones fall downwards because it is in their nature to do so has, as Molière noticed in the case of a similar explanation of why opium puts you to sleep, a hollow ring.