cannot know the difference between two things
unless we know both at the same time (De Anima iii, 2), and the same
is to be said of any other comparison. But our intellect knows the
difference and comparison between one thing and another. Therefore
it knows many things at the same time.

_On the contrary,_ It is said (Topic. ii, 10) that "understanding is
of one thing only, knowledge is of many."

_I answer that,_ The intellect can, indeed, understand many things
as one, but not as many: that is to say by _one_ but not by _many_
intelligible species. For the mode of every action follows the form
which is the principle of that action. Therefore whatever things the
intellect can understand under one species, it can understand at the
same time: hence it is that God sees all things at the same time,
because He sees all in one, that is, in His Essence. But whatever
things the intellect understands under different species, it does
not understand at the same time. The reason of this is that it is
impossible for one and the same subject to be perfected at the same
time by many forms of one genus and diverse species, just as it is
impossible for one and the same body at the same time to have
different colors or different shapes. Now all intelligible species
belong to one genus, because they are the perfections of one
intellectual faculty: although the things which the species represent
belong to different genera. Therefore it is impossible for one and
the same intellect to be perfected at the same time by different
intelligible species so as actually to understand different things.

Reply Obj. 1: The intellect is above that time, which is the measure
of the movement of corporeal things. But the multitude itself of
intelligible species causes a certain vicissitude of intelligible
operations, according as one operation succeeds another. And this
vicissitude is called time by Augustine, who says (Gen. ad lit. viii,
20, 22), that "God moves the spiritual creature through time."

Reply Obj. 2: Not only is it impossible for opposite forms to exist
at the same time in the same subject, but neither can any forms
belonging to the same genus, although they be not opposed to one
another, as is clear from the examples of colors and shapes.

Reply Obj. 3: Parts can be understood in two ways. First, in a
confused way, as existing in the whole, and thus they are known
through the one form of the whole, and so are known together. In
anot