Upon this I recurred to the definition which she had previously given of the soul, and I said that to my thinking her definition had not indicated distinctly enough all the powers of the soul which are a matter of observation. It declares the soul to be an intellectual essence which imparts to the organic body a force of life by which the senses operate. Now the soul is not thus operative only in our scientific and speculative intellect; it does not produce results in that world only, or employ the organs of sense only for this their natural work. On the contrary, we observe in our nature many emotions of desire and many of anger; and both these exist in us as qualities of our kind, and we see both of them in their manifestations displaying further many most subtle differences. There are many states, for instance, which are occasioned by desire; many others which on the other hand proceed from anger; and none of them are of the body; but that which is not of the body is plainly intellectual. Now our definition exhibits the soul as something intellectual; so that one of two alternatives, both absurd, must emerge when we follow out this view to this end; either anger and desire are both second souls in us, and a plurality of souls must take the place of the single soul, or the thinking faculty in us cannot be regarded as a soul either (if they are not), the intellectual element adhering equally to all of them and stamping them all as souls, or else excluding every one of them equally from the specific qualities of soul.
You are quite justified, she replied, in raising this question, and it has ere this been discussed by many elsewhere; namely, what we are to think of the principle of desire and the principle of anger within us. Are they consubstantial with the soul, inherent in the soul's very self from her first organization, or are they something different, accruing to us afterwards? In fact, while all equally allow that these principles are to be detected in the soul, investigation has not yet discovered exactly what we are to think of them so as to gain some fixed belief with regard to them. The generality of men still fluctuate in their opinions about this, which are as erroneous as they are numerous. As for ourselves, if the Gentile philosophy, which deals methodically with all these points, were really adequate for a demonstration, it would certainly be superfluous to add a discussion on the soul to those speculations. But while the latter proceeded, on the subject of the soul, as far in the direction of supposed consequences as the thinker pleased, we are not entitled to such licence, I mean that of affirming what we please; we make the Holy Scriptures the rule and the measure of every tenet; we necessarily fix our eyes upon that, and approve that alone which may be made to harmonize with the intention of those writings. We must therefore neglect the Platonic chariot and the pair of horses of dissimilar forces yoked to it, and their driver, whereby the philosopher allegorizes these facts about the soul; we must neglect also all that is said by the philosopher who succeeded him and who followed out probabilities by rules of art, and diligently investigated the very question now before us, declaring that the soul was mortal by reason of these two principles; we must neglect all before and since their time, whether they philosophized in prose or in verse, and we will adopt, as the guide of our reasoning, the Scripture, which lays it down as an axiom that there is no excellence in the soul which is not a property as well of the Divine nature. For he who declares the soul to be God's likeness asserts that anything foreign to Him is outside the limits of the soul; similarity cannot be retained in those qualities which are diverse from the original. Since, then, nothing of the kind we are considering is included in the conception of the Divine nature, one would be reasonable in surmising that such things are not consubstantial with the soul either. Now to seek to build up our doctrine by rule of dialectic and the science which draws and destroys conclusions, involves a species of discussion which we shall ask to be excused from, as being a weak and questionable way of demonstrating truth. Indeed, it is clear to every one that that subtle dialectic possesses a force that may be turned both ways, as well for the overthrow of truth as for the detection of falsehood; and so we begin to suspect even truth itself when it is advanced in company with such a kind of artifice, and to think that the very ingenuity of it is trying to bias our judgment and to upset the truth. If on the other hand any one will accept a discussion which is in a naked unsyllogistic form, we will speak upon these points by making our study of them so far as we can follow the chain of Scriptural tradition. What is it, then, that we assert? We say that the fact of the reasoning animal man being capable of understanding and knowing is most surely attested by those outside our faith; and that this definition would never have sketched our nature so, if it had viewed anger and desire and all such-like emotions as consubstantial with that nature. In any other case, one would not give a definition of the subject in hand by putting a generic instead of a specific quality; and so, as the principle of desire and the principle of anger are observed equally in rational and irrational natures, one could not rightly mark the specific quality by means of this generic one. But how can that which, in defining a nature, is superfluous and worthy of exclusion be treated as a part of that nature, and, so, available for falsifying the definition? Every definition of an essence looks to the specific quality of the subject in hand; and whatever is outside that speciality is set aside as having nothing to do with the required definition. Yet, beyond question, these faculties of anger and desire are allowed to be common to all reasoning and brute natures; anything common is not identical with that which is peculiar; it is imperative therefore that we should not range these faculties among those whereby humanity is exclusively meant: but just as one may perceive the principle of sensation, and that of nutrition and growth in man, and yet not shake thereby the given definition of his soul (for the quality A being in the soul does not prevent the quality B being in it too), so, when one detects in humanity these emotions of anger and desire, one cannot on that account fairly quarrel with this definition, as if it fell short of a full indication of man's nature.
What then, I asked the Teacher, are we to think about this? For I cannot yet see how we can fitly repudiate faculties which are actually within us.
Source: On the Soul and the Resurrection (New Advent)