5 These things might be treated at very great length, and our knowledge of the subject might be brought to greater certainty and fullness by a large induction from relevant facts. But take along with this letter the one which I sent you lately concerning images and memory, and study it somewhat more carefully; for it was manifest to me, from your reply, that it had not been fully understood. When, to the statements now before you, you add the portion of that letter in which I spoke of a certain natural faculty whereby the mind does in thought add to or take from any object as it pleases, you will see that it is possible for us both in dreams and in waking thoughts to conceive the images of bodily forms which we have never seen.
Source: Letters (New Advent)