I think, replied the Teacher, that I am myself responsible for this confusion arising from different accounts of the matter; for I did not state it as distinctly as I might have, by introducing a certain order of consequences for our consideration. Now, however, some such order shall, as far as it is possible, be devised, so that our essay may advance in the way of logical sequence and so give no room for such contradictions. We declare, then, that the speculative, critical, and world-surveying faculty of the soul is its peculiar property by virtue of its very nature, and that thereby the soul preserves within itself the image of the divine grace; since our reason surmises that divinity itself, whatever it may be in its inmost nature, is manifested in these very things—universal supervision and the critical discernment between good and evil. But all those elements of the soul which lie on the border-land and are capable from their peculiar nature of inclining to either of two opposites (whose eventual determination to the good or to the bad depends on the kind of use they are put to), anger, for instance, and fear, and any other such-like emotion of the soul divested of which human nature cannot be studied— all these we reckon as accretions from without, because in the Beauty which is man's prototype no such characteristics are to be found. Now let the following statement be offered as a mere exercise (in interpretation). I pray that it may escape the sneers of cavilling hearers. Scripture informs us that the Deity proceeded by a sort of graduated and ordered advance to the creation of man. After the foundations of the universe were laid, as the history records, man did not appear on the earth at once; but the creation of the brutes preceded his, and the plants preceded them. Thereby Scripture shows that the vital forces blended with the world of matter according to a gradation; first, it infused itself into insensate nature; and in continuation of this advanced into the sentient world; and then ascended to intelligent and rational beings. Accordingly, while all existing things must be either corporeal or spiritual, the former are divided into the animate and inanimate. By animate, I mean possessed of life: and of the things possessed of life, some have it with sensation, the rest have no sensation. Again, of these sentient things, some have reason, the rest have not. Seeing, then, that this life of sensation could not possibly exist apart from the matter which is the subject of it, and the intellectual life could not be embodied, either, without growing in the sentient, on this account the creation of man is related as coming last, as of one who took up into himself every single form of life, both that of plants and that which is seen in brutes. His nourishment and growth he derives from vegetable life; for even in vegetables such processes are to be seen when aliment is being drawn in by their roots and given off in fruit and leaves. His sentient organization he derives from the brute creation. But his faculty of thought and reason is incommunicable, and is a peculiar gift in our nature, to be considered by itself. However, just as this nature has the instinct acquisitive of the necessaries to material existence— an instinct which, when manifested in us men, we call Appetite— and as we admit this appertains to the vegetable form of life, since we can notice it there too like so many impulses working naturally to satisfy themselves with their kindred aliment and to issue in germination, so all the peculiar conditions of the brute creation are blended with the intellectual part of the soul. To them, she continued, belongs anger; to them belongs fear; to them all those other opposing activities within us; everything except the faculty of reason and thought. That alone, the choice product, as has been said, of all our life, bears the stamp of the Divine character. But since, according to the view which we have just enunciated, it is not possible for this reasoning faculty to exist in the life of the body without existing by means of sensations, and since sensation is already found subsisting in the brute creation, necessarily as it were, by reason of this one condition, our soul has touch with the other things which are knit up with it; and these are all those phænomena within us that we call “passions”; which have not been allotted to human nature for any bad purpose at all (for the Creator would most certainly be the author of evil, if in them, so deeply rooted as they are in our nature, any necessities of wrong-doing were found), but according to the use which our free will puts them to, these emotions of the soul become the instruments of virtue or of vice. They are like the iron which is being fashioned according to the volition of the artificer, and receives whatever shape the idea which is in his mind prescribes, and becomes a sword or some agricultural implement. Supposing, then, that our reason, which is our nature's choicest part, holds the dominion over these imported emotions (as Scripture allegorically declares in the command to men to rule over the brutes), none of them will be active in the ministry of evil; fear will only generate within us obedience, and anger fortitude, and cowardice caution; and the instinct of desire will procure for us the delight that is Divine and perfect. But if reason drops the reins and is dragged behind like a charioteer who has got entangled in his car, then these instincts are changed into fierceness, just as we see happens among the brutes. For since reason does not preside over the natural impulses that are implanted in them, the more irascible animals, under the generalship of their anger, mutually destroy each other; while the bulky and powerful animals get no good themselves from their strength, but become by their want of reason slaves of that which has reason. Neither are the activities of their desire for pleasure employed on any of the higher objects; nor does any other instinct to be observed in them result in any profit to themselves. Thus too, with ourselves, if these instincts are not turned by reasoning into the right direction, and if our feelings get the mastery of our mind, the man is changed from a reasoning into an unreasoning being, and from godlike intelligence sinks by the force of these passions to the level of the brute.
Much moved by these words, I said: To any one who reflects indeed, your exposition, advancing as it does in this consecutive manner, though plain and unvarnished, bears sufficiently upon it the stamp of correctness and hits the truth. And to those who are expert only in the technical methods of proof a mere demonstration suffices to convince; but as for ourselves, we were agreed that there is something more trustworthy than any of these artificial conclusions, namely, that which the teachings of Holy Scripture point to: and so I deem that it is necessary to inquire, in addition to what has been said, whether this inspired teaching harmonizes with it all.
Source: On the Soul and the Resurrection (New Advent)