3 R. Now I will have you answer me, does the soul seem to you to feel and perceive, or the body? A. The soul. R. And does the intellect appear to you to appertain to the soul? A. Assuredly. R. To the soul alone, or to something else? A. I see nothing else besides the soul, except God, in which I believe intellect to exist. R. Let us now consider that. If any one should tell you that wall was not a wall, but a tree, what would you think? A. Either that his senses or mine were astray, or that he called a wall by the name of a tree. R. What if he received in sense the image of a tree, and you of a wall?
May not both be true? A. By no means; because one and the same thing cannot be both a tree and a wall. For however individual things might appear different to us as individuals, it could not be but that one of us suffered a false imagination. R. What if it is neither tree nor wall, and you are both in error? A. That, indeed, is possible. R. This one thing therefore you had past by above. A. I confess it. R. What if you should acknowledge that anything seemed to you other than it is, are you then in error? A. No. R. Therefore that may be false which seems, and he not be in error to whom it seems. A. It may be so. R. It is to be allowed then that he is not in error who sees falsities, but he who assents to falsities. A. It is assuredly to be allowed. R. And this falsity, wherefore is it false? A. Because it is otherwise than it seems. R. If therefore there are none to whom it may seem, nothing is false. A. The inference is sound. R. Therefore the falsity is not in the things, but in the sense; but he is not beguiled who assents not to false things. It results that we are one thing, the sense another; since, when it is misled, we are able not to be misled. A. I have nothing to oppose to this. R. But when the soul is misled, do you venture to say that you are not false? A. How should I venture? R. But there is no sense without soul, no falsity without sense. Either therefore the soul operates, or cooperates with the falsity. A. Our preceding reasonings imply assent to this.
Source: Soliloquies (New Advent)