ere is
no other besides the five senses."

_I answer that,_ The reason of the distinction and number of the
senses has been assigned by some to the organs in which one or other
of the elements preponderate, as water, air, or the like. By others
it has been assigned to the medium, which is either in conjunction or
extrinsic and is either water or air, or such like. Others have
ascribed it to the various natures of the sensible qualities,
according as such quality belongs to a simple body or results from
complexity. But none of these explanations is apt. For the powers are
not for the organs, but the organs for the powers; wherefore there
are not various powers for the reason that there are various organs;
on the contrary, for this has nature provided a variety of organs,
that they might be adapted to various powers. In the same way nature
provided various mediums for the various senses, according to the
convenience of the acts of the powers. And to be cognizant of the
natures of sensible qualities does not pertain to the senses, but to
the intellect.

The reason of the number and distinction of the exterior senses must
therefore be ascribed to that which belongs to the senses properly and
_per se._ Now, sense is a passive power, and is naturally immuted by
the exterior sensible. Wherefore the exterior cause of such immutation
is what is _per se_ perceived by the sense, and according to the
diversity of that exterior cause are the sensitive powers diversified.

Now, immutation is of two kinds, one natural, the other spiritual.
Natural immutation takes place by the form of the immuter being
received according to its natural existence, into the thing immuted,
as heat is received into the thing heated. Whereas spiritual
immutation takes place by the form of the immuter being received,
according to a spiritual mode of existence, into the thing immuted, as
the form of color is received into the pupil which does not thereby
become colored. Now, for the operation of the senses, a spiritual
immutation is required, whereby an intention of the sensible form is
effected in the sensile organ. Otherwise, if a natural immutation
alone sufficed for the sense's action, all natural bodies would feel
when they undergo alteration.

But in some senses we find spiritual immutation only, as in _sight:_
while in others we find not only spiritual but also a natural
immutation; either on the part of the object only, or likewise on the
part