10 R. First let us again and yet again ventilate this question, What is falsity? A. I wonder if there will turn out to be anything, except what is not so as it seems. R. Give heed rather, and let us first question the senses themselves. For certainly what the eyes see, is not called false, unless it have some similitude of the true. For instance, a man whom we see in sleep, is not indeed a true man, but false, by this very fact that he has the similitude of a true one.
For who, seeing a dog, would have a right to say that he had dreamed of a man? Therefore too that is thereby a false dog, that it is like a true one. A. It is as you say. R. And moreover, if any one waking should see a horse and think he saw a man, is he not hereby misled, that there appears to him some similitude of a man? For if nothing should appear to him except the form of a horse, he cannot think that he sees a man. A. I fully concede this. R. We call that also a false tree which we see in a picture, and a false face which is reflected from a mirror, and a false motion of buildings to men that are sailing from them, and a false break in the oar when dipped, for no other reason than the verisimilitude in all these things. A. True. R. So we make mistakes between twins, so between eggs, so between seals stamped by one ring, and other such things. A. I follow and agree to all. R. Therefore that similitude of things which pertains to the eyes, is the mother of falsity. A. I cannot deny it.
Source: Soliloquies (New Advent)