17 A. Proceed, I pray; for now perchance you have begun to teach concerning falsities not falsely: but now I am considering of what sort that class of falsities may be, of which you have said, It tends to be, and is not. R. Why should you not consider? They are the same things, which already we have largely passed review. Does not your image in the mirror appear to will to be you yourself, but to be therefore false, because it is not? A. This does, in very deed, seem so. R. And as to pictures, and all such expressed resemblances, every such thing wrought by the artist?
Do they not press to be that, after whose similitude they have been made? A. I must certainly own this to be true. R. And you will allow, I believe, that the deceits under which dreamers, or madmen suffer, are to be included in this kind. A. None more: for none tend more to be such things as the waking and the sane discern; and yet they are hereby false, because that which they tend to be they cannot be. R. Why need I now say more concerning the gliding towers, or the dipped oar, or the shadows of bodies?
It is plain, as I think, that they are to be measured by this rule. A. Most evidently they are. R. I say nothing concerning the remaining senses; for no one by consideration will fail to find this, that in the various things which are subject to our sense, that is called false which tends to be anything and is not.
Source: Soliloquies (New Advent)