But first, what do I mean by “illusion,” and how does my meaning differ from “error” or “mistake” generally, and from “fallacy,” “delusion,” and “hallucination” in particular? I say “my meaning,” because the word is often used loosely (I do not say wrongly) for any of these synonyms: but I restrict it to a special sense.
“Illusion,” then, is wholesome error tending to the ultimate attainment of truth; “delusion” is harmful error arising from a perverted Imagination; “hallucination” is a wandering of the Imagination, without any guidance or support of fact, involving “delusion” of the most obstinate character; “fallacy” is an error of inference or reasoning; “mistake” is the result of mal-observation or weak memory; and “error” a general name for any deviation from the truth.
Illusion, in many cases, is an exaggerative and ornative tendency of the mind. It leads the very young to think their parents perfection, and the young to think them far better and wiser than they really are; it constrains the lover to exaggerate the beauty, accomplishments, and qualities of the woman whom he loves; it tends to the distortion of history by inclining all of us to accommodate facts to the wishes and preconceptions of our idealizing nature, which is always longing for “a more ample greatness, a more exact goodness, and a more absolute variety than can be found in the nature of things”;[[7]] and it lures us onward, young and old alike, over the rough places of life, even to the very brink of the grave, by the ever-fleeting, ever-reappearing suggestions of a bright to-morrow that shall make amends for the dull and commonplace to-day.
These illusive hopes, beliefs, and aspirations are never fulfilled in this life; but even the cynic and the pessimist must acknowledge, with Francis Bacon, that they constitute the very basis of all poetry that “tends to magnanimity and morality.” Those who believe in God will further recognize in illusion a divinely utilized integument for the preservation and development of aspirations that shall ultimately find a perfect fulfilment in a harmonious co-operation with the divine Love and in the unending contemplation of the divine Glory. Nor are illusions without a present practical purpose. Men are more hopeful, more active, more loving on account of them. On the other hand, even optimists must acknowledge that no man should shut his eyes to the truth in order to remain in what he knows to be no more than a comfortable error. The venial illusions of childhood, youth, and ignorance, become unpardonable or hypocritical in experienced age. Do you ask how we are to distinguish “illusions” from “delusions”? The answer is easy—on paper; but, in practice, often difficult to apply. However, the test is the same as that by which we distinguish knowledge from ignorance. Illusions “work”; that is to say, men are on the whole the better for them, and they prepare the way for truth. Delusions fail; men are in no way the better for them, and they often prepare the way for insanity and for physical or spiritual death.
We have spoken of moral illusions; let us touch on another kind of illusions to which some (I do not say rightly) have given the name of “illusions of sense.”
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?
They spring from a very strong conviction—erected upon the basis of Experience by Faith, but absolutely necessary for healthy life and spontaneous action—that the ordinary inferences which we almost instinctively derive from the report of the senses, are true, that is to say, will correspond to experience; and that we can act upon them without formally reasoning upon them.
Take the following instance. Shut your eyes, and get a friend to prick the back of your hand with the two points of a pair of compasses simultaneously, so that the two points may be about the eighth of an inch apart when they touch you; you will feel—and if you could not correct the inference by the sense of sight, you would infer—that only one point is pricking you. The reason is that the skin of the back of the hand only reports one sensation; and the mind leaps to the conclusion—owing to the multitude of past instances where one sensation has resulted from one object—that, in this instance also, one object alone is producing the sensation. A more curious instance is the following: Place the middle finger over the first finger, and between the two fingers thus interlaced place a single marble or your nose: you will appear to be touching two marbles or two noses. The reason is this: when the two fingers are in their usual position (not thus interlaced) and touching marbles or similar objects, two simultaneous sensations on the right side of the right finger and on the left side of the left finger would always imply two marbles; now you have constrained the two fingers to assume an unusual position where these two simultaneous sensations can be produced by one marble; but you, following custom, would infer the presence of two marbles, if sight, or other evidence, did not shew there was only one.
But illusions from the sense of touch are far less common than illusions from the sense of sight. We all know how a cloud or sheet or coal may be converted by the Imagination into an image of something entirely different and visible only to the imaginer, although he supposes that others “must see it” too. But these are, so to speak, private illusions: the great public and, at one time, universal illusion, was the conviction that the sun and the stars move and that the earth does not move. There is scarcely any illusion more natural than this. Our senses give no indication whatever of the earth’s motion; but they do indicate that the sun and the stars are moving. So complicated a process of reasoning, and so much experience, are needed before a man can realize (as distinct from repeating on authority) the causes for believing in the earth’s motion that it is by no means surprising that, even now, only a minority of the human race believe that they are dashing through space at the rate of some thousands of miles an hour; and, except during the last three hundred years, the illusion that the earth is at rest was universal. Another common illusion from sight is that which leads us to suppose that, when we see anything in the air, a straight line from our eye towards the image which we see would touch the object itself: whereas, in reality, the image is raised by refraction so that in misty weather we see an object considerably higher than it is, and I suppose (to speak with strict exactness) we never “see” an object precisely where it is.