I answer that, A power as such is directed to an act. Wherefore we seek to know the nature of a power from the act to which it is directed, and consequently the nature of a power is diversified, as the nature of the act is diversified. Now the nature of an act is diversified according to the various natures of the objects. For every act is either of an active power or of a passive power. Now, the object is to the act of a passive power, as the principle and moving cause: for color is the principle of vision, inasmuch as it moves the sight. On the other hand, to the act of an active power the object is a term and end; as the object of the power of growth is perfect quantity, which is the end of growth. Now, from these two things an act receives its species, namely, from its principle, or from its end or term; for the act of heating differs from the act of cooling, in this, that the former proceeds from something hot, which is the active principle, to heat; the latter from something cold, which is the active principle, to cold. Therefore the powers are of necessity distinguished by their acts and objects.
Nevertheless, we must observe that things which are accidental do not change the species. For since to be colored is accidental to an animal, its species is not changed by a difference of color, but by a difference in that which belongs to the nature of an animal, that is to say, by a difference in the sensitive soul, which is sometimes rational, and sometimes otherwise. Hence "rational" and "irrational" are differences dividing animal, constituting its various species. In like manner therefore, not any variety of objects diversifies the powers of the soul, but a difference in that to which the power of its very nature is directed. Thus the senses of their very nature are directed to the passive quality which of itself is divided into color, sound, and the like, and therefore there is one sensitive power with regard to color, namely, the sight, and another with regard to sound, namely, hearing. But it is accidental to a passive quality, for instance, to something colored, to be a musician or a grammarian, great or small, a man or a stone. Therefore by reason of such differences the powers of the soul are not distinct.