Since some things are and are not, without coming to be and ceasing to be, eg points, if they can be said to be, and in general forms (for it is not white that comes to be, but the wood comes to be white, if everything that comes to be comes from something and comes to be something), not all contraries can come from one another, but it is in different senses that a white man comes from a black man, and white comes from black.