Master: I shall now demonstrate the existence of the simple substances according to the impression of the substances upon each other by the analytical method, although by the synthetic method it is quite evident. I ask you first for the two principles that you require in order to understand this.
Pupil: What are they?
Master: Do you grant that the body in itself is at rest and inactive?
Pupil: I would not say otherwise if I did not see simple bodies, such as fire, air, and water, each of which moves in place.
Master: Since the motion of each of them does not come from the fact that they are bodies, but from the fact that they are qualities by the qualities with which they are endowed, you must know that their motions do not in any way prevent the body from being in itself at rest and inactive.
Pupil: What proof is there that the motions of the elements do not come from the fact that the elements are bodies?
Master: If the motions of the elements came from the fact that the elements are bodies, they would not be different.
Pupil: Why so?
Master: If they came from the essence of the body, their motion would be one because the body is one.