se things
without which they are unable to think of any nature--i.e. imaginary
pictures of corporeal things." Therefore the nature of the human
intellect is not only incorporeal, but it is also a substance, that
is, something subsistent.

_I answer that,_ It must necessarily be allowed that the principle of
intellectual operation which we call the soul, is a principle both
incorporeal and subsistent. For it is clear that by means of the
intellect man can have knowledge of all corporeal things. Now whatever
knows certain things cannot have any of them in its own nature;
because that which is in it naturally would impede the knowledge of
anything else. Thus we observe that a sick man's tongue being vitiated
by a feverish and bitter humor, is insensible to anything sweet, and
everything seems bitter to it. Therefore, if the intellectual
principle contained the nature of a body it would be unable to know
all bodies. Now every body has its own determinate nature. Therefore
it is impossible for the intellectual principle to be a body. It is
likewise impossible for it to understand by means of a bodily organ;
since the determinate nature of that organ would impede knowledge of
all bodies; as when a certain determinate color is not only in the
pupil of the eye, but also in a glass vase, the liquid in the vase
seems to be of that same color.

Therefore the intellectual principle which we call the mind or the
intellect has an operation _per se_ apart from the body. Now only that
which subsists can have an operation _per se._ For nothing can operate
but what is actual: for which reason we do not say that heat imparts
heat, but that what is hot gives heat. We must conclude, therefore,
that the human soul, which is called the intellect or the mind, is
something incorporeal and subsistent.

Reply Obj. 1: "This particular thing" can be taken in two senses.
Firstly, for anything subsistent; secondly, for that which subsists,
and is complete in a specific nature. The former sense excludes the
inherence of an accident or of a material form; the latter excludes
also the imperfection of the part, so that a hand can be called "this
particular thing" in the first sense, but not in the second.
Therefore, as the human soul is a part of human nature, it can indeed
be called "this particular thing," in the first sense, as being
something subsistent; but not in the second, for in this sense, what
is composed of body and soul is said to be "th