Different Modes of Conceiving
But since the eye of the mind cannot look at all things together, in one glance, which the memory retains, these trinities of thought alternate in a series of withdrawals and successions, and so that trinity becomes most innumerably numerous; and yet not infinite, if it pass not beyond the number of things stored up in the memory. For, although we begin to reckon from the earliest perception which any one has of material things through any bodily sense, and even take in also those things which he has forgotten, yet the number would undoubtedly be certain and determined, although innumerable. For we not only call infinite things innumerable, but also those, which, although finite, exceed any one's power of reckoning.
13. But we can hence perceive a little more clearly that what the memory stores up and retains is a different thing from that which is thence copied in the conception of the man who remembers, although, when both are combined together, they appear to be one and the same; because we can only remember just as many species of bodies as we have actually seen, and so great, and such, as we have actually seen; for the mind imbibes them into the memory from the bodily sense; whereas the things seen in conception, although drawn from those things which are in the memory, yet are multiplied and varied innumerably, and altogether without end. For I remember, no doubt, but one sun, because according to the fact, I have seen but one; but if I please, I conceive of two, or three, or as many as I will; but the vision of my mind, when I conceive of many, is formed from the same memory by which I remember one. And I remember it just as large as I saw it. For if I remember it as larger or smaller than I saw it, then I no longer remember what I saw, and so I do not remember it. But because I remember it, I remember it as large as I saw it; yet I conceive of it as greater or as less according to my will. And I remember it as I saw it; but I conceive of it as running its course as I will, and as standing still where I will, and as coming whence I will, and whither I will. For it is in my power to conceive of it as square, although I remember it as round; and again, of what color I please, although I have never seen, and therefore do not remember, a green sun; and as the sun, so all other things. But owing to the corporeal and sensible nature of these forms of things, the mind falls into error when it imagines them to exist without, in the same mode in which it conceives them within, either when they have already ceased to exist without, but are still retained in the memory, or when in any other way also, that which we remember is formed in the mind, not by faithful recollection, but after the variations of thought.
14. Yet it very often happens that we believe also a true narrative, told us by others, of things which the narrators have themselves perceived by their senses. And in this case, when we conceive the things narrated to us, as we hear them, the eye of the mind does not seem to be turned back to the memory, in order to bring up visions in our thoughts; for we do not conceive these things from our own recollection, but upon the narration of another; and that trinity does not here seem to come to its completion, which is made when the species lying hid in the memory, and the vision of the man that remembers, are combined by will as a third. For I do not conceive that which lay hid in my memory, but that which I hear, when anything is narrated to me. I am not speaking of the words themselves of the speaker, lest any one should suppose that I have gone off to that other trinity, which is transacted without, in sensible things, or in the senses: but I am conceiving of those species of material things, which the narrator signifies to me by words and sounds; which species certainly I conceive of not by remembering, but by hearing. But if we consider the matter more carefully, even in this case, the limit of the memory is not overstepped. For I could not even understand the narrator, if I did not remember generically the individual things of which he speaks, even although I then hear them for the first time as connected together in one tale. For he who, for instance, describes to me some mountain stripped of timber, and clothed with olive trees, describes it to me who remembers the species both of mountains, and of timber, and of olive trees; and if I had forgotten these, I should not know at all of what he was speaking, and therefore could not conceive that description. And so it comes to pass, that every one who conceives things corporeal, whether he himself imagine anything, or hear, or read, either a narrative of things past, or a foretelling of things future, has recourse to his memory, and finds there the limit and measure of all the forms at which he gazes in his thought. For no one can conceive at all, either a color or a form of body, which he never saw, or a sound which he never heard, or a flavor which he never tasted, or a scent which he never smelt, or any touch of a corporeal thing which he never felt. But if no one conceives anything corporeal except what he has [sensuously] perceived, because no one remembers anything corporeal except what he has thus perceived, then, as is the limit of perceiving in bodies, so is the limit of thinking in the memory. For the sense receives the species from that body which we perceive, and the memory from the sense; but the mental eye of the concipient, from the memory.
Source: On the Holy Trinity (New Advent)