presupposes something immovable: for
when a change of quality occurs, the substance remains unmoved; and
when there is a change of substantial form, matter remains unmoved.
Moreover the various conditions of mutable things are themselves
immovable; for instance, though Socrates be not always sitting, yet
it is an immovable truth that whenever he does sit he remains in one
place. For this reason there is nothing to hinder our having an
immovable science of movable things.
_______________________

SECOND ARTICLE [I, Q. 84, Art. 2]

Whether the Soul Understands Corporeal Things Through Its Essence?

Objection 1: It would seem that the soul understands corporeal things
through its essence. For Augustine says (De Trin. x, 5) that the soul
"collects and lays hold of the images of bodies which are formed in
the soul and of the soul: for in forming them it gives them something
of its own substance." But the soul understands bodies by images of
bodies. Therefore the soul knows bodies through its essence, which it
employs for the formation of such images, and from which it forms
them.

Obj. 2: Further, the Philosopher says (De Anima iii, 8) that "the
soul, after a fashion, is everything." Since, therefore, like is known
by like, it seems that the soul knows corporeal things through itself.

Obj. 3: Further, the soul is superior to corporeal creatures. Now
lower things are in higher things in a more eminent way than in
themselves, as Dionysius says (Coel. Hier. xii). Therefore all
corporeal creatures exist in a more excellent way in the soul than in
themselves. Therefore the soul can know corporeal creatures through
its essence.

_On the contrary,_ Augustine says (De Trin. ix, 3) that "the mind
gathers knowledge of corporeal things through the bodily senses." But
the soul itself cannot be known through the bodily senses. Therefore
it does not know corporeal things through itself.

_I answer that,_ The ancient philosophers held that the soul knows
bodies through its essence. For it was universally admitted that
"like is known by like." But they thought that the form of the thing
known is in the knower in the same mode as in the thing known. The
Platonists however were of a contrary opinion. For Plato, having
observed that the intellectual soul has an immaterial nature, and an
immaterial mode of knowledge, held that the forms of things known
subsist immaterially. While the earlier natural philosophers,
observing that thing