of the organ. On the part of the object we find natural
immutation, as to place, in sound which is the object of _hearing;_
for sound is caused by percussion and commotion of air: and we find
natural immutation by alteration, in odor which is the object of
_smelling;_ for in order to exhale an odor, a body must be in a
measure affected by heat. On the part of an organ, natural immutation
takes place in _touch_ and _taste;_ for the hand that touches
something hot becomes hot, while the tongue is moistened by the
humidity of the flavored morsel. But the organs of smelling and
hearing are not affected in their respective operations by any
natural immutation unless indirectly.

Now, the sight, which is without natural immutation either in its
organ or in its object, is the most spiritual, the most perfect, and
the most universal of all the senses. After this comes the hearing and
then the smell, which require a natural immutation on the part of the
object; while local motion is more perfect than, and naturally prior
to, the motion of alteration, as the Philosopher proves (Phys. viii,
7). Touch and taste are the most material of all: of the distinction
of which we shall speak later on (ad 3, 4). Hence it is that the three
other senses are not exercised through a medium united to them, to
obviate any natural immutation in their organ; as happens as regards
these two senses.

Reply Obj. 1: Not every accident has in itself a power of immutation
but only qualities of the third species, which are the principles of
alteration: therefore only suchlike qualities are the objects of the
senses; because "the senses are affected by the same things whereby
inanimate bodies are affected," as stated in Phys. vii, 2.

Reply Obj. 2: Size, shape, and the like, which are called "common
sensibles," are midway between "accidental sensibles" and "proper
sensibles," which are the objects of the senses. For the proper
sensibles first, and of their very nature, affect the senses; since
they are qualities that cause alteration. But the common sensibles
are all reducible to quantity. As to size and number, it is clear
that they are species of quantity. Shape is a quality about quantity,
since the notion of shape consists of fixing the bounds of magnitude.
Movement and rest are sensed according as the subject is affected in
one or more ways in the magnitude of the subject or of its local
distance, as in the movement of growth or of locomotion, or ag