19 I am asking, I am speaking remember of a distant resemblance. So let no one say, See what he has compared to God! I have advertised you of this already, and by anticipation have both put you on your guard, and have guarded myself. The two are indeed very far removed from each other, as the lowest from the Highest, as the changeable from the Unchangeable, the created from the Creator, the human nature from the Divine. Lo! I apprise you of this at first, that no one may say ought against me, because there is so great a difference in the things whereof I am about to speak. Lest then while I am asking for your ears, you should any of you be getting ready your teeth, remember I have undertaken merely to show, that there are some three things which are separately exhibited, whose operation is yet inseparable. How like or how unlike these things are to the Almighty Trinity is no concern of mine at present; but in the very creatures of the lowest order, and subject to change, we do find three things which may be separately exhibited, whose operation is yet inseparable. O carnal imagination! Obstinate, unbelieving conscience! Why as concerning that ineffable Majesty do you doubt as to that thing, which you can discover in your own self? For I ask you, O man, have you memory? If not, how have you retained what I have said? But perhaps you have forgotten already what I said but a little while ago. Yet these very words, “I said”— these two syllables, you could not retain except by memory. For how should you know they were two, if as the second sounded, you had forgotten the first? But why do I dwell longer on this? Why am I so urgent? Why do I so press conviction? For you have memory; it is plain. I am searching then for something else. Have you understanding? “I have,” you will say. For had you not memory, you could not retain what I said; and had you not understanding, you could not comprehend what you have retained. You have then this as well as the other. You recall your understanding unto that which you retain within, and so you see it, and by seeing are fashioned into that state as to be said to know. But I am searching for a third thing. Memory you have, whereby to retain what is said; and understanding you have, whereby to understand what is retained; but as touching these two, I ask again of you, Have you not with your will retained and understood? Undoubtedly, with my will, you will say. So then you have will.
These are the three things which I promised I would bring home to your ears and minds. These three things are in you, which you can number, but canst not separate. These three then, memory, understanding, and will— these three, I say, consider how they are separately exhibited, yet is their operation inseparable.
Source: Sermons on Selected Lessons of the New Testament (New Advent)