Book 11
3
Now since there is a science of nature, clearly it must be different from both practical and productive science. In a productive science the source of motion is in the producer and not in the thing produced, and is either an art or some other kind of potency; and similarly in a practical science the motion is not in the thing acted upon but rather in the agent.
4
But the science of the natural philosopher is concerned with things which contain in themselves a source of motion. From this it is clear that natural science must be neither practical nor productive, but speculative; since it must fall under one of these classes.
5
And since every science must have some knowledge of the essence and must use it as a starting-point, we must be careful to observe how the natural philosopher should define, and how he should regard the formula of essence—whether in the same way as the term snub, or rather as the term concave.
6
For of these the formula of snub is stated in conjunction with the matter of the object, whereas that of concave is stated apart from the matter; since snubness is only found in the nose, which is therefore included in the formula, for the snub is a concave nose . Thus it is obvious that the formula of flesh and eye and the other parts of the body must always be stated in conjunction with their matter.
