Objection 1. It would seem that the image of God is not found in the acts of the soul. For Augustine says (De Civ. Dei xi, 26), that "man was made to God's image, inasmuch as we exist and know that we exist, and love this existence and knowledge." But to exist does not signify an act. Therefore the image of God is not to be found in the soul's acts.
Objection 2. Further, Augustine (De Trin. ix, 4) assigns God's image in the soul to these three things--mind, knowledge, and love. But mind does not signify an act, but rather the power or the essence of the intellectual soul. Therefore the image of God does not extend to the acts of the soul.
Objection 3. Further, Augustine (De Trin. x, 11) assigns the image of the Trinity in the soul to "memory, understanding, and will." But these three are "natural powers of the soul," as the Master of the Sentences says (1 Sent. D iii). Therefore the image of God is in the powers, and does not extend to the acts of the soul.
Objection 4. Further, the image of the Trinity always remains in the soul. But an act does not always remain. Therefore the image of God does not extend to the acts.
On the contrary, Augustine (De Trin. xi, 2 seqq.) assigns the trinity in the lower part of the soul, in relation to the actual vision, whether sensible or imaginative. Therefore, also, the trinity in the mind, by reason of which man is like to God's image, must be referred to actual vision.
I answer that, As above explained (2), a certain representation of the species belongs to the nature of an image. Hence, if the image of the Divine Trinity is to be found in the soul, we must look for it where the soul approaches the nearest to a representation of the species of the Divine Persons. Now the Divine Persons are distinct from each other by reason of the procession of the Word from the Speaker, and the procession of Love connecting Both. But in our soul word "cannot exist without actual thought," as Augustine says (De Trin. xiv, 7). Therefore, first and chiefly, the image of the Trinity is to be found in the acts of the soul, that is, inasmuch as from the knowledge which we possess, by actual thought we form an internal word; and thence break forth into love. But, since the principles of acts are the habits and powers, and everything exists virtually in its principle, therefore, secondarily and consequently, the image of the Trinity may be considered as existing in the powers, and still more in the habits, forasmuch as the acts virtually exist therein.
Reply to Objection 1. Our being bears the image of God so far as if is proper to us, and excels that of the other animals, that is to say, in so far as we are endowed with a mind. Therefore, this trinity is the same as that which Augustine mentions (De Trin. ix, 4), and which consists in mind, knowledge, and love.
Reply to Objection 2. Augustine observed this trinity, first, as existing in the mind. But because the mind, though it knows itself entirely in a certain degree, yet also in a way does not know itself--namely, as being distinct from others (and thus also it searches itself, as Augustine subsequently proves--De Trin. x, 3,4); therefore, as though knowledge were not in equal proportion to mind, he takes three things in the soul which are proper to the mind, namely, memory, understanding, and will; which everyone is conscious of possessing; and assigns the image of the Trinity pre-eminently to these three, as though the first assignation were in part deficient.
Reply to Objection 3. As Augustine proves (De Trin. xiv, 7), we may be said to understand, will, and to love certain things, both when we actually consider them, and when we do not thing of them. When they are not under our actual consideration, they are objects of our memory only, which, in his opinion, is nothing else than habitual retention of knowledge and love [Cf. 79, 7, ad 1]. "But since," as he says, "a word cannot be there without actual thought (for we think everything that we say, even if we speak with that interior word belonging to no nation's tongue), this image chiefly consists in these three things, memory, understanding, and will. And by understanding I mean here that whereby we understand with actual thought; and by will, love, or dilection I mean that which unites this child with its parent." From which it is clear that he places the image of the Divine Trinity more in actual understanding and will, than in these as existing in the habitual retention of the memory; although even thus the image of the Trinity exists in the soul in a certain degree, as he says in the same place. Thus it is clear that memory, understanding, and will are not three powers as stated in the Sentences.
Reply to Objection 4. Someone might answer by referring to Augustine's statement (De Trin. xiv, 6), that "the mind ever remembers itself, ever understands itself, ever loves itself"; which some take to mean that the soul ever actually understands, and loves itself. But he excludes this interpretation by adding that "it does not always think of itself as actually distinct from other things." Thus it is clear that the soul always understands and loves itself, not actually but habitually; though we might say that by perceiving its own act, it understands itself whenever it understands anything. But since it is not always actually understanding, as in the case of sleep, we must say that these acts, although not always actually existing, yet ever exist in their principles, the habits and powers. Wherefore, Augustine says (De Trin. xiv, 4): "If the rational soul is made to the image of God in the sense that it can make use of reason and intellect to understand and consider God, then the image of God was in the soul from the beginning of its existence."
Source: Summa Theologica (New Advent)