Aristotle. Aristotle said in Book Lambda, which represents the culmination of his philosophy—namely, the science of metaphysics:
“As for considering matters by way of analogy, if we follow what we have set forth, it is most fitting for us to explain that the principles of all existing things are three: the element, form, and privation. An example of this in sensible substance is that heat corresponds to form, cold corresponds to privation, and the element is that which potentially possesses both. In the category of quality, whiteness corresponds to form, blackness to privation, and the underlying subject of both—the surface—corresponds to the element. Likewise, light corresponds to form, darkness to privation, and the body capable of receiving light is the underlying subject of both. Thus, in an unqualified sense, it is impossible to find particular elements that are themselves the elements of all things; but by way of analogy and comparison, such elements are more likely to be found.”
He said:
“Our present inquiry is not a search for the element of existing things; rather, our aim is to seek their principle. Both are causes of them, except that (1) a principle may exist outside the thing, as in the case of the moving cause, whereas elements can exist only within the things composed of them. There is no [impediment] (2) preventing whatever is an element from being called a principle, but whatever is a principle does not necessarily have an element.
“This is because a moving principle may exist outside that which it moves. However, (3) the proximate mover of natural things is like form. This is because—”