a human being is begotten only by another human being. As for things produced by thought, the moving cause is either the form or privation, as in the art of medicine and ignorance of it, [and the art of building and ignorance of it] (1). In many cases, the moving cause is the form. For example, the art of medicine is, in a certain respect, health, because health is the moving cause; and the form of a house is, in a certain respect, the art of building. Likewise, a human being is begotten only by another human being (2).
Our aim, however, is not to seek the proximate mover, but the First Mover from which all things derive their motion. It is plainly a substance, for it is the principle of substances (3), and the principle of substances (4) can only be a substance. It is the principle of substances (5) and the principle of all existing things. The earlier reluctance to state this explicitly was unwarranted, for all other things are merely occurrences, states, and motions of substance.
We must investigate what this substance that moves the entire body is: must we posit that it is a soul, an intellect, or something other than either? We must first take care not to attribute to the First Principle any of the accidents that attach to the lowest ranks of existing things. Among the lowest ranks of existing things, however, there may be that which exists potentially; a thing may assume different states at different times and may not always remain in a single state. Things susceptible to generation and corruption are those that exist in this manner, for in them you find the very same thing existing at one time potentially and at another actually.