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Objection 1: It would seem that our intellect can know immaterial
substances through the knowledge of material things. For Dionysius
says (Coel. Hier. i) that "the human mind cannot be raised up to
immaterial contemplation of the heavenly hierarchies, unless it is led
thereto by material guidance according to its own nature." Therefore
we can be led by material things to know immaterial substances.
Objection 2: Further, science resides in the intellect. But there
are sciences and definitions of immaterial substances; for Damascene
defines an angel (De Fide Orth. ii, 3); and we find angels
treated of both in theology and philosophy. Therefore immaterial
substances can be understood by us.
Objection 3: Further, the human soul belongs to the genus of
immaterial substances. But it can be understood by us through its act
by which it understands material things. Therefore also other material
substances can be understood by us, through their material effects.
Objection 4: Further, the only cause which cannot be comprehended
through its effects is that which is infinitely distant from them, and
this belongs to God alone. Therefore other created immaterial
substances can be understood by us through material things.
On the contrary, Dionysius says (Div. Nom. i) that
"intelligible things cannot be understood through sensible things, nor
composite things through simple, nor incorporeal through corporeal."
I answer that, Averroes says (De Anima iii) that a philosopher
named Avempace taught that by the understanding of natural substances
we can be led, according to true philosophical principles, to the
knowledge of immaterial substances. For since the nature of our
intellect is to abstract the quiddity of material things from matter,
anything material residing in that abstracted quiddity can again be made
subject to abstraction; and as the process of abstraction cannot go on
forever, it must arrive at length at some immaterial quiddity,
absolutely without matter; and this would be the understanding of
immaterial substance.
Now this opinion would be true, were immaterial substances the forms
and species of these material things; as the Platonists supposed.
But supposing, on the contrary, that immaterial substances differ
altogether from the quiddity of material things, it follows that
however much our intellect abstract the quiddity of material things from
matter, it could never arrive at anything akin to immaterial
substance. Therefore we are not able perfectly to understand
immaterial substances through material substances.
Reply to Objection 1: From material things we can rise to some kind
of knowledge of immaterial things, but not to the perfect knowledge
thereof; for there is no proper and adequate proportion between
material and immaterial things, and the likenesses drawn from material
things for the understanding of immaterial things are very dissimilar
therefrom, as Dionysius says (Coel. Hier. ii).
Reply to Objection 2: Science treats of higher things principally
by way of negation. Thus Aristotle (De Coel. i, 3) explains
the heavenly bodies by denying to them inferior corporeal properties.
Hence it follows that much less can immaterial substances be known by
us in such a way as to make us know their quiddity; but we may have a
scientific knowledge of them by way of negation and by their relation to
material things.
Reply to Objection 3: The human soul understands itself through its
own act of understanding, which is proper to it, showing perfectly its
power and nature. But the power and nature of immaterial substances
cannot be perfectly known through such act, nor through any other
material thing, because there is no proportion between the latter and
the power of the former.
Reply to Objection 4: Created immaterial substances are not in the
same natural genus as material substances, for they do not agree in
power or in matter; but they belong to the same logical genus, because
even immaterial substances are in the predicament of substance, as
their essence is distinct from their existence. But God has no
connection with material things, as regards either natural genus or
logical genus; because God is in no genus, as stated above
(Question 3, Article 5). Hence through the likeness derived
from material things we can know something positive concerning the
angels, according to some common notion, though not according to the
specific nature; whereas we cannot acquire any such knowledge at all
about God.
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