Now up till this point in De Trinitate VIII, he insists that God is nothing less than Truth itself and Goodness itself and that one can attain to the knowledge of God through understanding Truth and Goodness. But there is yet a more excellent way by which we might attain to the knowledge of God, and that is through Love.
So while we would have innitially thought that Augustine would have found a Trinitarian analogy in Truth or Goodness, he does not . At least not directly. Rather, he finds it in Love which is a speicies of the Good. And Love, unlike Truth or Goodness, is not merely some abstract notion that the mind apprehends, but it is really an act that the mind performs. This in turn opens the doorway to his other trinitarian analogies.
But what is love or charity, which divine Scripture so greatly praises and proclaims, except the love of the good? But love is of some one that loves, and with love something is loved. Behold, then, there are three things: he that loves, and that which is loved, and love. What, then, is love, except a certain life which couples or seeks to couple together some two things, namely, him that loves, and that which is loved? And this is so even in outward and carnal loves. But that we may drink in something more pure and clear, let us tread down the flesh and ascend to the mind. What does the mind love in a friend except the mind? There, then, also are three things: he that loves, and that which is loved, and love. It remains to ascend also from hence, and to seek those things which are above, as far as is given to man.
But here for a little while let our purpose rest, not that it may think itself to have found already what it seeks; but just as usually the place has first to be found where anything is to be sought, while the thing itself is not yet found, but we have only found already where to look for it; so let it suffice to have said thus much, that we may have, as it were, the hinge of some starting-point, whence to weave the rest of our discourse.
De Trinitate VIII. 10. 14