I grant, it is true, that no man is deceived in so far as he perceives; that is to say, I grant that mental images considered in themselves involve no error; but I deny that a man in so far as he perceives affirms nothing. For what else is it to perceive a winged horse than to affirm of the horse that it has wings? For if the mind perceived nothing else but this winged horse, it would regard it as present, nor would it have any reason for doubting its existence, nor any power of refusing assent to it, unless the image of the winged horse be joined to an idea which negates its existence, or the mind perceives that the idea of the winged horse which it has is inadequate. In either of the two latter cases it will necessarily deny or doubt the existence of the horse. With regard to the third objection, what has been said will perhaps be a sufficient answer--namely, that the will is something universal, which is predicated of all ideas, and that it signifies that only which is common to them all, that is to say, affirmation. Its adequate essence, therefore, in so far as it is thus considered in the abstract, must be in every idea, and in this sense only must it be the same in all; but not in so far as it is considered as constituting the essence of an idea, for so far, the individual affirmations differ just as the ideas differ. For example, the affirmation which the idea of a circle involves differs from that which the idea of a triangle involves, just as the idea of a circle differs from the idea of a triangle. Again, I absolutely deny that we need a power of thinking in order to affirm that to be true which is true, equal to that which we need in order to affirm that to be true which is false. For these two affirmations, if we look to the mind, are related to one another as being and non-being, for there is nothing positive in ideas which constitutes a form of falsity. Here therefore particularly is it to be observed how easily we are deceived when we confuse universals with individuals, and the entities of reason and abstractions with realities. With regard to the fourth objection, I say that I entirely grant that if a man were placed in such a state of equilibrium he would perish of hunger and thirst, supposing he perceived nothing but hunger and thirst, and the food and drink which were equidistant from him. If you ask me whether such a man would not be thought an ass rather than a man, I reply that I do not know; nor do I know what ought to be thought of a man who hangs himself, or of children, fools, and madmen. _The Independence of Mind and Body_