Next, we have to note that the word "substance" itself has two senses. When we spoke of substance as one of the categories we were using it in a secondary sense. We meant by substances "horse," "man," and the rest of the "real kinds" which we find in Nature, and try to reproduce in a scientific classification. In this sense of the word "substances" are a special class of _predicates_, as when we affirm of Plato that he is a man, or of Bucephalus that he is a horse. But in the primary sense a substance means an absolutely individual thing, "_this_ man," or "_this_ horse." We may therefore define primary substances from the logician’s point of view by saying that they can be only subjects of predication, never predicates. Or again, it is peculiar to substances, that while remaining numerically one a substance admits of incompatible determinations, as Socrates, remaining one and the same Socrates, is successively young and old. This is not true of "qualities," "actions," and the rest. The same colour cannot be first white and then black; the same act cannot be first bad and then good. Thus we may say that individual substances are the fixed and permanent factors in the world of mutability, the invariants of existence. Processes go on in them, they run the gamut of changes from birth to decay, processes take place _among_ them, they act on and are acted on by one another, they fluctuate in their qualities and their magnitude, but so long as a substance exists it remains numerically one and the same throughout all these changes. Their existence is the first and most fundamental condition of the existence of the universe, since they are the bearers of all qualities, the terms of all relations, and the agents and patients in all interaction. The point to note is that Aristotle begins his investigation into the structure of What Is and the causes by which it is produced by starting from the existence of individual things belonging to the physical order and perceived by the senses. About any such thing we may ask two questions, (1) into what constituent factors can it be logically analysed? (2) and how has it come to exhibit the character which our analysis shows it to have? The answer to these questions will appear from a consideration of two standing antitheses which run through Aristotle’s philosophy, the contrast between Matter and Form, and that between Potential and Actual, followed by a recapitulation of his doctrine of the Four Causes, or four senses of the word Cause.