On The Holy Trinity by St. Augustine
Chapter 22.--How Great the Unlikeness is Between the Image of the Trinity Which We Have Found in Ourselves, and the Trinity Itself.
42. But since these are in one person, as man is, some one may say to us, These three things, memory, understanding, and love, are mine, not their own; neither do they do that which they do for themselves, but for me, or rather I do it by them. For it is I who remember by memory, and understand by understanding, and love by love: and when I direct the mind's eye to my memory, and so say in my heart the thing I know, and a true word is begotten of my knowledge, both are mine, both the knowledge certainly and the word. For it is I who know, and it is I who say in my heart the thing I know. And when I come to find in my memory by thinking that I understand and love anything, which understanding and love were there also before I thought thereon, it is my own understanding and my own love that I find in my own memory, whereby it is I that understand, and I that love, not those things themselves. Likewise, when my thought is mindful, and wills to return to those things which it had left in the memory, and to understand and behold them, and say them inwardly, it is my own memory that is mindful, and it is my own, not its will, wherewith it wills. When my very love itself, too, remembers and understands what it ought to desire and what to avoid, it remembers by my, not by its own memory; and understands that which it intelligently loves by my, not by its own, understanding. In brief, by all these three things, it is I that remember, I that understand, I that love, who am neither memory, nor understanding, nor love, but who have them. These things, then, can be said by a single person, which has these three, but is not these three. But in the simplicity of that Highest Nature, which is God, although there is one God, there are three persons, the Father, the Son, and the Holy Spirit.