Whether our intellect can understand immaterial substances through its knowledge of material things?
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 [*Ibn-Badja, Arabian Philosopher; ob. 1183] 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 (Q[3], A[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.
Whether God is the first object known by the human mind?
Objection 1: It would seem that God is the first object known by the human mind. For that object in which all others are known, and by which we judge others, is the first thing known to us; as light is to the eye, and first principles to the intellect. But we know all things in the light of the first truth, and thereby judge of all things, as Augustine says (De Trin. xii, 2; De Vera Relig. xxxi; [*Confess. xii, 25]). Therefore God is the first object known to us.
Objection 2: Further, whatever causes a thing to be such is more so. But God is the cause of all our knowledge; for He is "the true light which enlighteneth every man that cometh into this world" ( Jn. 1:9 ). Therefore God is our first and most known object.
Objection 3: Further, what is first known in the image is the exemplar to which it is made. But in our mind is the image of God, as Augustine says (De Trin. xii, 4,7). Therefore God is the first object known to our mind.
On the contrary, "No man hath seen God at any time" ( Jn. 1:18 ).
I answer that, Since the human intellect in the present state of life cannot understand even immaterial created substances (A[1]), much less can it understand the essence of the uncreated substance. Hence it must be said simply that God is not the first object of our knowledge. Rather do we know God through creatures, according to the Apostle ( Rom. 1:20 ), "the invisible things of God are clearly seen, being understood by the things that are made": while the first object of our knowledge in this life is the "quiddity of a material thing," which is the proper object of our intellect, as appears above in many passages (Q[84], A[7]; Q[85], A[8]; Q[87], A[2], ad 2)
Reply to Objection 1: We see and judge of all things in the light of the first truth, forasmuch as the light itself of our mind, whether natural or gratuitous, is nothing else than the impression of the first truth upon it, as stated above (Q[12], A[2]). Hence, as the light itself of our intellect is not the object it understands, much less can it be said that God is the first object known by our intellect.
Reply to Objection 2: The axiom, "Whatever causes a thing to be such is more so," must be understood of things belonging to one and the same order, as explained above (Q[81], A[2], ad 3). Other things than God are known because of God; not as if He were the first known object, but because He is the first cause of our faculty of knowledge.
Reply to Objection 3: If there existed in our souls a perfect image of God, as the Son is the perfect image of the Father, our mind would know God at once. But the image in our mind is imperfect; hence the argument does not prove.