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In questions eighty-four to eighty-eight of the first
part of the "Summa theologica", St. Thomas
treats only of the acts and habits of the intellective part
of the soul, because the acts and habits of the appetitive
part are considered in moral theology and because the
operations of the sensitive part do not directly pertain to
theology. St. Thomas asks: 1. how the soul joined to
the body understands corporeal things (q. 84); by
what means it knows them (q. 85); what it understands
in them (q. 86); 2. how the soul knows itself and
the things that are in itself (q. 87); 3. how the
soul knows the things that are above, that is, immaterial
substances (q. 88).
It should be noted particularly that for St. Thomas the
adequate object of our intellect, as intellect, is
intelligible being in the entire extent of being. Hence
we are able to know God naturally as the first cause, and
supernaturally we can be elevated to the direct vision of
the divine essence, which is not outside the full extent
of being.[1320]
But the proper or proportionate object of the human
intellect, as human, is the essence of sensible things,
since the lowest intelligible being of sensible things,
knowable by means of the senses, corresponds to the lowest
intellect. Hence our intellect is united to the
senses.[1321] Hence also we know God and spiritual
substances naturally only by analogy, in the mirror of
sensible things. In the state of union with the body our
souls do not know spiritual things directly as does the
angel, and therefore it conceives spiritual being as
immaterial, and this is a sign that the soul first knows
the nature of material things, such as the nature of
stones, plants, and animals.
In particular it is asked whether the soul as united to
the body knows itself through its essence. In "De
veritate"[1322] St. Thomas examines the
arguments pro and con at great length, and in the
"Summa theologica"[1323] he proceeds in a
simpler way and says: "Whatever is knowable is knowable
as it is in act..... For sight does not perceive the
colored thing in potency but only in act. And so it is
with the intellect..... Thus it is that we do not
know prime matter except in its relation to the form.
Hence in immaterial substances, just as each one is in
act by its essence so each one is intelligible by its
essence..... God, who is pure act and from whom all
things proceed, not only knows Himself but all things
through His essence. The essence of the angel is in the
genus of intelligible being as it is act, but not pure
act..... Hence the angel knows itself through its
essence, but the angel does not know everything through
its essence; it knows some things through their
representations. The position of the human intellect in
the scale of intelligible beings is that of a being in
potency, similar to the position of prime matter in the
scale of sensible being, and therefore the human intellect
is called possibilis. Considered in its essence,
therefore, the human intellect is a cognitive potency.
Of itself it has the power of intellection but it does not
have the power of being known except when it is in act.
But because it is connatural for our intellect in its
present state to be concerned with material and sensible
things, it follows that our intellect knows itself
inasmuch as it is in act by means of the species abstracted
from sensible things by the light of the intellectus
agens, which is the act of these intelligible beings, and
through the mediation of these intelligible species the
intellectus possibilis understands. Our intellect
therefore knows itself not through its essence but by its
act.[1324]
This happens in two ways. First, in the particular when
Socrates or Plato perceives that he has an intellective
soul from the fact that he perceives that he understands.
Secondly, in the universal when we study the human mind
through the act of the intellect. But it is true that the
efficacy of this knowledge, by which we understand the
nature of the soul, is based on the light which our
intellect derives from divine truth, in which the natures
of all things are contained.
St. Thomas therefore arrives at the same conclusion that
he reached in the "De veritate": "Hence our
mind cannot understand itself in the sense that it
understands itself directly or immediately."[1325]
If the soul knew itself immediately through its own
essence, its spirituality would be fully evident to the
soul, and there would be no materialists, just as there
are no materialists among the angels. But when the soul
is separated from the body, in the exact instant of the
separation when the soul is no longer existing in the
body, the soul will know itself through itself.
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