[II.]— He Asks What the Great Knowledge is that Victor Imparts
Therefore, brother greatly beloved, I desire to know what you learned of him, in order that, if I have already possessed the knowledge, I may participate in your joy; but if I happen to be ignorant, I may be instructed by you. Did you not then understand that there are two somethings, soul and spirit, according as it is said in Scripture, “You will separate my soul from my spirit”? And that both of them pertain to man's nature, so that the whole man consists of spirit, and soul, and body?
Sometimes, however, these two are combined together under the designation of soul; for instance, in the passage, “And man became a living soul.” Now, in this place the spirit is implied. Similarly in sundry passages the two are described under the name of spirit, as when it is written, “And He bowed His head and gave up the spirit;” in which passage it is the soul that must also be understood. And that the two are of one and the same substance? I suppose that you already knew all this.
But if you did not, then you may as well know that you have not acquired any great knowledge, the ignorance of which would be attended with much danger. And if there must be any more subtle discussion on such points it would be better to carry on the controversy with himself, whose wordy qualities we have already discovered. The questions we might consider are: whether, when mention is made of the soul, the spirit is also implied in the term in such a way that the two comprise the soul, the spirit being, as it were, some part of it—whether, in fact (as this person seemed to think), under the designation soul, the whole is so designated from only a part; or else, whether the two together make up the spirit, that which is properly called soul being a part thereof; whether again, in fact, the whole is not called from only a part, when the term spirit is used in such a wide sense as to comprehend the soul also, as this man supposes.
These, however, are but subtle distinctions, and ignorance about them certainly is not attended with any great danger.
Source: On the Soul and its Origin (New Advent)