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Objection 1: It would seem that intellectual knowledge is not
derived from sensible things. For Augustine says (Questions.
83, qu. 9) that "we cannot expect to learn the fulness of truth
from the senses of the body." This he proves in two ways. First,
because "whatever the bodily senses reach, is continually being
changed; and what is never the same cannot be perceived." Secondly,
because, "whatever we perceive by the body, even when not present to
the senses, may be present to the imagination, as when we are asleep
or angry: yet we cannot discern by the senses, whether what we
perceive be the sensible object or the deceptive image thereof. Now
nothing can be perceived which cannot be distinguished from its
counterfeit." And so he concludes that we cannot expect to learn the
truth from the senses. But intellectual knowledge apprehends the
truth. Therefore intellectual knowledge cannot be conveyed by the
senses.
Objection 2: Further, Augustine says (Gen. ad lit. xii,
16): "We must not thing that the body can make any impression on
the spirit, as though the spirit were to supply the place of matter in
regard to the body's action; for that which acts is in every way more
excellent than that which it acts on." Whence he concludes that "the
body does not cause its image in the spirit, but the spirit causes it
in itself." Therefore intellectual knowledge is not derived from
sensible things.
Objection 3: Further, an effect does not surpass the power of its
cause. But intellectual knowledge extends beyond sensible things: for
we understand some things which cannot be perceived by the senses.
Therefore intellectual knowledge is not derived from sensible things.
On the contrary, The Philosopher says (Metaph. i, 1; Poster.
ii, 15) that the principle of knowledge is in the senses.
I answer that, On this point the philosophers held three opinions.
For Democritus held that "all knowledge is caused by images issuing
from the bodies we think of and entering into our souls," as
Augustine says in his letter to Dioscorus (cxviii, 4). And
Aristotle says (De Somn. et Vigil.) that Democritus held that
knowledge is cause by a "discharge of images." And the reason for
this opinion was that both Democritus and the other early philosophers
did not distinguish between intellect and sense, as Aristotle relates
(De Anima iii, 3). Consequently, since the sense is affected by
the sensible, they thought that all our knowledge is affected by this
mere impression brought about by sensible things. Which impression
Democritus held to be caused by a discharge of images.
Plato, on the other hand, held that the intellect is distinct from
the senses: and that it is an immaterial power not making use of a
corporeal organ for its action. And since the incorporeal cannot be
affected by the corporeal, he held that intellectual knowledge is not
brought about by sensible things affecting the intellect, but by
separate intelligible forms being participated by the intellect, as we
have said above (Articles 4,5). Moreover he held that sense is a
power operating of itself. Consequently neither is sense, since it is
a spiritual power, affected by the sensible: but the sensible organs
are affected by the sensible, the result being that the soul is in a
way roused to form within itself the species of the sensible.
Augustine seems to touch on this opinion (Gen. ad lit. xii, 24)
where he says that the "body feels not, but the soul through the
body, which it makes use of as a kind of messenger, for reproducing
within itself what is announced from without." Thus according to
Plato, neither does intellectual knowledge proceed from sensible
knowledge, nor sensible knowledge exclusively from sensible things;
but these rouse the sensible soul to the sentient act, while the senses
rouse the intellect to the act of understanding.
Aristotle chose a middle course. For with Plato he agreed that
intellect and sense are different. But he held that the sense has not
its proper operation without the cooperation of the body; so that to
feel is not an act of the soul alone, but of the "composite." And
he held the same in regard to all the operations of the sensitive part.
Since, therefore, it is not unreasonable that the sensible objects
which are outside the soul should produce some effect in the
"composite," Aristotle agreed with Democritus in this, that the
operations of the sensitive part are caused by the impression of the
sensible on the sense: not by a discharge, as Democritus said, but
by some kind of operation. For Democritus maintained that every
operation is by way of a discharge of atoms, as we gather from De
Gener. i, 8. But Aristotle held that the intellect has an
operation which is independent of the body's cooperation. Now nothing
corporeal can make an impression on the incorporeal. And therefore in
order to cause the intellectual operation according to Aristotle, the
impression caused by the sensible does not suffice, but something more
noble is required, for "the agent is more noble than the patient,"
as he says (De Gener. i, 5). Not, indeed, in the sense that
the intellectual operation is effected in us by the mere intellectual
operation is effected in us by the mere impression of some superior
beings, as Plato held; but that the higher and more noble agent which
he calls the active intellect, of which we have spoken above
(Question 79, Articles 3,4) causes the phantasms received from
the senses to be actually intelligible, by a process of abstraction.
According to this opinion, then, on the part of the phantasms,
intellectual knowledge is caused by the senses. But since the
phantasms cannot of themselves affect the passive intellect, and
require to be made actually intelligible by the active intellect, it
cannot be said that sensible knowledge is the total and perfect cause of
intellectual knowledge, but rather that it is in a way the material
cause.
Reply to Objection 1: Those words of Augustine mean that we must
not expect the entire truth from the senses. For the light of the
active intellect is needed, through which we achieve the unchangeable
truth of changeable things, and discern things themselves from their
likeness.
Reply to Objection 2: In this passage Augustine speaks not of
intellectual but of imaginary knowledge. And since, according to the
opinion of Plato, the imagination has an operation which belongs to
the soul only, Augustine, in order to show that corporeal images are
impressed on the imagination, not by bodies but by the soul, uses the
same argument as Aristotle does in proving that the active intellect
must be separate, namely, because "the agent is more noble than the
patient." And without doubt, according to the above opinion, in the
imagination there must needs be not only a passive but also an active
power. But if we hold, according to the opinion of Aristotle, that
the action of the imagination, is an action of the "composite,"
there is no difficulty; because the sensible body is more noble than
the organ of the animal, in so far as it is compared to it as a being
in act to a being in potentiality; even as the object actually colored
is compared to the pupil which is potentially colored. It may,
however, be said, although the first impression of the imagination is
through the agency of the sensible, since "fancy is movement produced
in accordance with sensation" (De Anima iii, 3), that
nevertheless there is in man an operation which by synthesis and
analysis forms images of various things, even of things not perceived
by the senses. And Augustine's words may be taken in this sense.
Reply to Objection 3: Sensitive knowledge is not the entire cause
of intellectual knowledge. And therefore it is not strange that
intellectual knowledge should extend further than sensitive knowledge.
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