Chapter 1
We have treated of that which is primarily and to which all the other categories of being are referred — i.e. of substance. For it is in virtue of the concept of substance that the others also are said to be — quantity and quality and the like; for all will be found to involve the concept of substance, as we said in the first part of our work.[1] And since 'being' is in one way divided into 'what', quality, and quantity, and is in another way distinguished in respect of potency and complete reality, and of function, let us discuss potency and complete reality. First let us explain potency in the strictest sense, which is, however, not the most useful[2] for our present purpose. For potency and actuality extend further than the mere sphere of motion. But when we have spoken of this first kind, we shall in our discussions of actuality[3] explain the other kinds of potency.
We have pointed out elsewhere[4] that 'potency' and the word 'can' have several senses. Of these we may neglect all the potencies that are so called by an equivocation. For some are called so by analogy, as in geometry; and[5] we say things can be or cannot be because in some definite way they are or are not.
But all potencies that conform to the same type are starting-points, and are called potencies in reference to one primary kind of potency, which is a starting-point of change in another thing or in the thing itself qua other. For one kind is a potency of being acted on, i.e. the principle in the very thing acted on, which makes it capable of being changed and acted on by another thing or by itself regarded as other; and another kind
