6 And again, in order that he may draw the sound from that sounding-board below, he addresses his soul: he says, “Why are you sorrowful, O my soul, and why do you disquiet me?”. I am in tribulations, in weariness, in mourning, “Why do you disquiet me, O my soul?” Who is the speaker, to whom is he speaking? That it is the soul to which he is speaking, everybody knows: for it is obvious: the appeal is addressed to it directly: “Why are you sorrowful, O my soul, and why do you disquiet me?”
The question is as to the speaker. It is not the flesh addressing the soul, surely, since the flesh cannot speak without the soul. For it is more appropriate for the soul to address the flesh, than for the flesh to address the soul....We perceive then that we have a certain part, in which is “the image of God;” viz. the mind and reason. It was that same mind that prayed for “God's Light” and “God's Truth.” It is the same mind by which we apprehend right and wrong: it is by the same that we discern truth from falsehood.
It is this same that we call “understanding;” which “understanding,” indeed, is wanting to the brutes. And this “understanding” whoever neglects in himself, and holds it in less account than the other parts of his nature, and casts it off, just as if he had it not, is addressed in the Psalm, “Be not as the horse and the mule, which have no understanding.” It is our “understanding” then that is addressing our soul. The latter is withered away from tribulations, worn out in anguish, made “sorrowful” in temptations, fainting in toils. The mind, catching a glimpse of Truth above, would fain rouse her spirits, and she says, “Why are you sorrowful, O my soul?”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)