De Anima

But since, as in the whole of nature, to something which serves as matter for each kind (and this is potentially all the passive members of the kind) there corresponds something else which is the cause or agent because it makes them all, the two being related to one another as art to its material, of necessity these differences must be found also in the soul. And to the one intellect, which answers to this description because it becomes all things, corresponds the other because it makes all things, like a sort of definite quality such as light. For in a manner light, too, converts colours which are potential into actual colours. And it is this intellect which is separable and impassive and unmixed, being in its essential nature an activity. For that which acts is always superior to that which is acted upon, the cause or principle to the matter. Now actual knowledge is identical with the thing known, but potential knowledge is prior in time in the individual; and yet not universally prior in time. But this intellect has no intermittence in its thought. It is, however, only when separated that it is its true self, and this, its essential nature, alone is immortal and eternal. But we do not remember because this is impassive, while the intellect which can be affected is perishable and without this does not think at all.