Different Modes of Conceiving
15 Further, as the will applies the sense to the bodily object, so it applies the memory to the sense, and the eye of the mind of the concipient to the memory. But that which harmonizes those things and unites them, itself also disjoins and separates them, that is, the will. But it separates the bodily senses from the bodies that are to be perceived, by movement of the body, either to hinder our perceiving the thing, or that we may cease to perceive it: as when we avert our eyes from that which we are unwilling to see, or shut them; so, again, the ears from sounds, or the nostrils from smells.
So also we turn away from tastes, either by shutting the mouth, or by casting the thing out of the mouth. In touch, also, we either remove the bodily thing, that we may not touch what we do not wish, or if we were already touching it, we fling or push it away. Thus the will acts by movement of the body, so that the bodily sense shall not be joined to the sensible things. And it does this according to its power; for when it endures hardship in so doing, on account of the condition of slavish mortality, then torment is the result, in such wise that nothing remains to the will save endurance.
But the will averts the memory from the sense; when, through its being intent on something else, it does not suffer things present to cleave to it. As any one may see, when often we do not seem to ourselves to have heard some one who was speaking to us, because we were thinking of something else. But this is a mistake; for we did hear, but we do not remember, because the words of the speaker presently slipped out of the perception of our ears, through the bidding of the will being diverted elsewhere, by which they are usually fixed in the memory.
Therefore, we should say more accurately in such a case, we do not remember, than, we did not hear; for it happens even in reading, and to myself very frequently, that when I have read through a page or an epistle, I do not know what I have read, and I begin it again. For the purpose of the will being fixed on something else, the memory was not so applied to the bodily sense, as the sense itself was applied to the letters. So, too, any one who walks with the will intent on something else, does not know where he has got to; for if he had not seen, he would not have walked there, or would have felt his way in walking with greater attention, especially if he was passing through a place he did not know; yet, because he walked easily, certainly he saw; but because the memory was not applied to the sense itself in the same way as the sense of the eyes was applied to the places through which he was passing, he could not remember at all even the last thing he saw.
Now, to will to turn away the eye of the mind from that which is in the memory, is nothing else but not to think thereupon.
Source: On the Holy Trinity (New Advent)