A short analysis of the principal subjects treated of in the Metaphysics of Aristotle will serve to exhibit the details of the former method, as far as our present limits will permit, the unchanging principle of all change and motion. Sensible substances, the objects of physical science, are subject to change; and all change implies a progress of the same subject from one of two opposite states to the other; from not being to being, or the reverse. Hence change implies three elements: the form, the privation, and the matter potentially susceptible of both.
But change itself must take place in consequence of some cause; we must therefore add a fourth principle to the three elements. We are thus led to the notion of a substance which is the efficient cause of change, and this substance must be eternal; for even change and time are conceived as imperishable, and these depend upon a substance.
The cause of change must therefore be a being eternally acting, and which, consequently, cannot be conceived as having a power to act prior to the exercise of that power. In other words, the first cause can never be potentially that which it is not actually. The first cause is thus active without being passive; it moves all things without being itself moved.
The action of an unmoved cause of motion may be regarded as analogous to that of an object of desire on the appetite, or of an object of contemplation on the intellect; for these excite to action without being themselves acted upon. Thus the principle of change may be conceived both as first cause and as final cause or chief good, which all things desire, and by the desire of which they are moved.
***Excerpt from Henry Longueville Mansel, B.d.. Metaphysics or the philosophy of consciousness