I doubt whether the name is correctly given; for to me it seems that the illusion proceeds not from the senses (which, as far as I can judge, never deceive us) but from the imaginations and inferences which we base upon the report of the senses. Take an extreme case, fit rather to be called “delusion” than “illusion.” If I see the phantom of a cat before the fire, which cat nobody else in the room can see, do my senses deceive me? No; but I am deceived by the imaginative inference which leads me to assume from past experience that the object which I see is visible to, and can be touched by, everybody else. My visual sense (which has to do with images only) reports—and can do no otherwise—that it discerns the image of a cat. That report is true. But then my imagination forces on me the belief that this is an ordinary tangible and visible cat. That belief is false. Or take again the not infrequent case of colour-blindness. I am a signalman, and cannot tell a green light from a red: do my senses deceive me when I call a red light green? No; my sense reports inadequately for my necessities, and coarsely is compared with those who possess a finer sense of colour, but not deceitfully. My error arises from having loosely and servilely used the distinctive words “red” and “green” from childhood to manhood, although my senses continually protested that they could not distinguish two colours corresponding to the two words: but I imagined that there must be some such distinction for the two, and that I must be capable of recognizing it, because everybody around me recognized it. If we are to say that the signalman’s senses deceive him we must be prepared to admit that every man’s senses deceive him more or less. Do you suppose, when you see anything, that you see that which the thing _is_? “This is a yellowish-green,” say you. “Of course,” a Superior Being might reply; “but which of the one hundred and fifty shades of yellowish-green is it? You might as well tell me, when I shew you a sheep, ‘This is a _being_,’ as tell me simply this is ‘yellowish-green.’” We do not see things as Superior Beings see them; but we are not on that account to say that our sight deceives us. Our visual sense reports the truth more or less adequately: but our Imagination, prompted by insufficient experience and inference, leads us sometimes to illusive conclusions. Still, although “illusions of sense” ought perhaps to be rather called “illusions _from_ sense;”—_i.e._ illusions arising “from” the report of the senses, but not illusions in which the senses are themselves deceived—no one will deny that such illusions exist. Sometimes they are exceptional, but sometimes so common as to be almost universal. Let us enumerate a few and ask whence they spring, and what purpose they serve?