What, then, is justice? We distinctly remember what in the course of our argument we said in the commencement of this treatise; namely, that man was fashioned in imitation of the Divine nature, preserving his resemblance to the Deity as well in other excellences as in possession of freedom of the will, yet being of necessity of a nature subject to change. For it was not possible that a being who derived his origin from an alteration should be altogether free from this liability.
For the passing from a state of non-existence into that of existence is a kind of alteration; when being, that is, by the exercise of Divine power takes the place of nonentity. In the following special respect, too, alteration is necessarily observable in man, namely, because man was an imitation of the Divine nature, and unless some distinctive difference had been occasioned, the imitating subject would be entirely the same as that which it resembles; but in this instance, it is to be observed, there is a difference between that which “was made in the image” and its pattern; namely this, that the one is not subject to change, while the other is (for, as has been described, it has come into existence through an alteration), and being thus subject to alteration does not always continue in its existing state.
For alteration is a kind of movement ever advancing from the present state to another; and there are two forms of this movement; the one being ever towards what is good, and in this the advance has no check, because no goal of the course to be traversed can be reached, while the other is in the direction of the contrary, and of it this is the essence, that it has no subsistence; for, as has been before stated, the contrary state to goodness conveys some such notion of opposition, as when we say, for instance, that that which is is logically opposed to that which is not, and that existence is so opposed to non-existence.
Since, then, by reason of this impulse and movement of changeful alteration it is not possible that the nature of the subject of this change should remain self-centred and unmoved, but there is always something towards which the will is tending, the appetency for moral beauty naturally drawing it on to movement, this beauty is in one instance really such in its nature, in another it is not so, only blossoming with an illusive appearance of beauty; and the criterion of these two kinds is the mind that dwells within us.
Under these circumstances it is a matter of risk whether we happen to choose the real beauty, or whether we are diverted from its choice by some deception arising from appearance, and thus drift away to the opposite; as happened, we are told in the heathen fable, to the dog which looked askance at the reflection in the water of what it carried in its mouth, but let go the real food, and, opening its mouth wide to swallow the image of it, still hungered. Since, then, the mind has been disappointed in its craving for the real good, and diverted to that which is not such, being persuaded, through the deception of the great advocate and inventor of vice, that that was beauty which was just the opposite (for this deception would never have succeeded, had not the glamour of beauty been spread over the hook of vice like a bait),— the man, I say, on the one hand, who had enslaved himself by indulgence to the enemy of his life, being of his own accord in this unfortunate condition—I ask you to investigate, on the other hand, those qualities which suit and go along with our conception of the Deity, such as goodness, wisdom, power, immortality, and all else that has the stamp of superiority. As good, then, the Deity entertains pity for fallen man; as wise He is not ignorant of the means for his recovery; while a just decision must also form part of that wisdom; for no one would ascribe that genuine justice to the absence of wisdom.
Source: The Great Catechism (New Advent)