Since just as in the whole nature on the one hand there is the matter of each kind of thing (and this is all those things in potentiality), while on the other the cause and productive, in that it produces all these things, e.g. as art stands to its material, so too in the case of the soul there must be these differences.
And there is such an intellect as to become all things, and again another such as to produce all things, as a sort of ability, like light; for in some way light too makes the potential colors into actual colors. And this intellect is distinct, unaffected and unmixed, and in respect of its nature activity.
For that which acts is always superior to that which is acted upon, and the origin (the origin of movement) to the matter. And actual knowledge is similar to its object, while potential knowledge is prior in time in each one of us, but, in general, it is not prior in time.
The active intellect
Nor yet is it that sometimes thinks (the intellect) and sometimes not. On the other hand, it is only when it is distinct that it is just what it is, and only this is immortal and eternal. We do not remember because this (the active intellect) is unaffected, whereas the passive intellect is perishable, and without this things nothing.
Bibliography: Aristotle De anima (430a.10 to 430a.25)
Translation: George Kotsalis