e under the one power of common sense. Therefore the powers are
not distinguished according to the difference of their objects.

_On the contrary,_ Things that are subsequent are distinguished by
what precedes. But the Philosopher says (De Anima ii, 4) that "acts
and operations precede the powers according to reason; and these
again are preceded by their opposites," that is their objects.
Therefore the powers are distinguished according to their acts and
objects.

_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 anothe