Perception is a species of a wider genus, namely sensitivity. Sensitivity is not confined to living things; in fact it is best exemplified by scientific instruments. A material object is said to be “sensitive” to such and such a stimulus, if, when the stimulus is present, it behaves in a way noticeably different from that in which it behaves in the absence of the stimulus. A photographic plate is sensitive to light, a barometer is sensitive to pressure, a thermometer to temperature, a galvanometer to electric current, and so on. In all these cases, we might say, in a certain metaphorical sense, that an instrument “perceives” the stimulus to which it is sensitive. We do not in fact say so; we feel that perception involves something more than we find in scientific instruments. What is this something more?
The traditional answer would be: consciousness. But this answer, right or wrong, is not what we are seeking at the moment, because we are considering the percipient as he appears to an outside observer, to whom his “consciousness” is only an inference. Is there anything in perception as viewed from without that distinguishes it from the sensitivity of a scientific instrument?
There is, of course, the fact that human beings are sensitive to a greater variety of stimuli than any instrument. Each separate sense-organ can be surpassed by something made artificially sensitive to its particular stimulus. Photographic plates can photograph stars that we cannot see; clinical thermometers register differences of temperature that we cannot feel; and so on. But there is no way of combining a microscope, a microphone, a thermometer, a galvanometer, and so on, into a single organism which will react in an integral manner to the combination of all the different stimuli that affect its different “sense-organs”. This, however, is perhaps only a proof that our mechanical skill is not so great as it may in time become. It is certainly not enough to define the difference between a dead instrument and a living body.
The chief difference—perhaps the only one from our present point of view—is that living bodies are subject to the law of association or of the “conditioned reflex”. Consider, for instance, an automatic machine. It has a reflex which makes it sensitive to pennies, in response to which it gives up chocolate. But it never learns to give up chocolate on merely seeing a penny, or hearing the word “penny”. If you kept it in your house, and said “Abracadabra” to it every time you inserted a penny, it would not in the end be moved to action by the mere word “Abracadabra”. Its reflexes remain unconditioned, as do some of ours, such as sneezing. But with us sneezing is peculiar in this respect—hence its unimportance. Most of our reflexes can be conditioned, and the conditioned reflex can in turn be conditioned afresh, and so on without limit. This is what makes the reactions of the higher animals, and especially of man, so much more interesting and complicated than the reactions of machines. Let us see whether this one law will suffice to distinguish perception from other forms of sensitivity.
The variability in a human being’s responses to a given stimulus has given rise to the traditional distinction between cognition and volition. When one’s rich uncle comes for a visit, smiles are the natural response; after he has lost his money, a colder demeanour results from the new conditioning. Thus the reaction to the stimulus has come to be divided into two parts, one purely receptive and sensory, the other active and motor. Perception, as traditionally conceived, is, so to speak, the end term of the receptive-sensory part of the reaction, while volition (in its widest sense) is the first term of the active-motor part of the reaction. It was possible to suppose that the receptive part of the reaction would be always the same for the same stimulus, and that the difference due to experience would only arise in the motor part. The last term of the passive part, as it appears to the person concerned, was called “sensation”. But in fact the influence of the law of conditioned reflexes goes much deeper than this theory supposed. As we saw, the contraction of the pupil, which is normally due to bright light, can be conditioned so as to result from a loud noise. What we see depends largely upon muscular adjustments of the eyes, which we make quite unconsciously. But apart from the contraction of the pupil only one of them is a true reflex, namely turning the eyes towards a bright light. This is a movement which children can perform on the day of their birth; I know this, not merely from personal observation, but also, what is more, from the text-books. But new-born infants cannot follow a moving light with their eyes, nor can they focus or accommodate. As a consequence, the purely receptive part of their reaction to visual objects, in so far as this reaction is visual, is different from that of adults or older children, whose eye muscles adjust themselves so as to see clearly.
But here again all sorts of factors enter in. Innumerable objects are in our field of vision, but only some (at most) are interesting to us. If some one says “look, there’s a snake”, we adjust our eyes afresh and obtain a new “sensation”. Then, when the purely visual part is finished, there are stimulations, by association, of other centres in the brain. There are pictures, in Köhler’s book, of apes watching other apes on the top of insecure piles of boxes, and the spectators have their arms raised in sympathetic balancing movements. Any one who watches gymnastics or skilful dancing is liable to experience sympathetic muscular contractions. Any visual object that we might be touching will stimulate incipient touch reactions, but the sun, moon, and stars do not.
Conversely, visual reactions may be stimulated through association with other stimuli. When motor-cars were still uncommon, I was walking one day with a friend when a tire punctured in our neighbourhood with a loud report. He thought it was a revolver, and averred that he had seen the flash. In dreams, this sort of mechanism operates uncontrolled. Some stimulus—say the noise of the maid knocking at the door—becomes interpreted in fantastic ways which are governed by association. I remember once dreaming that I was in an inn in the country in Germany and was wakened by a choir singing outside my window. Finally I really woke, and found that a spring shower was making a very musical noise on the roof. At least, I heard a very musical noise, and now re-interpreted it as a shower on the roof. This hypothesis I confirmed by looking out of the window. In waking life we are critical of the interpretative hypotheses that occur to us, and therefore do not make such wild mistakes as in dreams. But the creative, as opposed to the critical, mechanism is the same in waking life as it is in dreams: there is always far more richness in the experience than the sensory stimulus alone would warrant. All adaptation to environment acquired during the life of the individual might be regarded as learning to dream dreams that succeed rather than dreams that fail. The dreams we have when we are asleep usually end in a surprise: the dreams we have in waking life are less apt to do so. Sometimes they do, as when pride goes before a fall; but in that case they are regarded as showing maladjustment, unless there is some large external cause, such as an earthquake. One might say that a person properly adapted to his environment is one whose dreams never end in the sort of surprise that would wake him up. In that case, he will think that his dreams are objective reality. But if modern physics is to be believed, the dreams we call waking perceptions have only a very little more resemblance to objective reality than the fantastic dreams of sleep. They have some truth, but only just so much as is required to make them useful.
Until we begin to reflect, we unhesitatingly assume that what we see is really “there” in the outside world, except in such cases as reflections in mirrors. Physics and the theory of the way in which perceptions are caused show that this naive belief cannot be quite true. Perception may, and I think does, enable us to know something of the outer world, but it is not the direct revelation that we naturally suppose it to be. We cannot go into this question adequately until we have considered what the philosopher has to learn from physics; I am merely giving, by anticipation, the reasons for regarding perception as a form of reaction to the environment, displayed in some bodily movement, rather than as a form of knowledge. When we have considered further what constitutes knowledge, we may find that perception is, after all, a form of knowledge, but only because knowledge is not quite what we naturally suppose it to be. For the present, let us stick to the view of perception that can be obtained by the external observer, i.e. as something displayed in the manner of reacting to the environment.
From the point of view of the external observer, perception is established just like any other causal correlation. We observe that, whenever a certain object stands in a certain spatial relation to a man’s body, the man’s body makes a certain movement or set of movements; we shall then say that the man “perceives” the object. So the new-born baby turns its eyes slowly towards a bright light which is not in the centre of the field of vision; this entitles us to say that the baby “perceives” the light. If he is blind, his eyes do not move in this way. A bird flying about in a wood does not bump into the branches, whereas in a room it will bump into the glass of the window. This entitles us to say that the bird perceives the branches but not the glass. Do we “perceive” the glass or do we merely know that it is there? This question introduces us to the complications produced by association. We know by experience, from the sense of touch, that there is usually glass in window-frames; thus it makes us react to the window-frames as if we could see the glass. But sometimes there is no glass, and still we shall perhaps behave as if there were. If this can happen, it shows that we do not perceive the glass, since our reaction is the same whether there is glass or not. If, however, the glass is coloured, or slightly distorting, or not perfectly clean, a person accustomed to glass will be able to distinguish a frame containing glass from one which has none. In that case it is more difficult to decide whether we are to say that he “perceives” the glass or not. It is certain that perception is affected by experience. A person who can read perceives print where another would not. A musician perceives differences between notes which to an untrained ear are indistinguishable. People unaccustomed to the telephone cannot understand what they hear in it; but this is perhaps not really a case in point.
The difficulty we are considering arises from the fact that a human body, unlike a scientific instrument, is perpetually changing its reaction to a given stimulus, under the influence of the law of association. Moreover, the human body is always doing something. How, then, are we to know whether what it is doing is the result of a given stimulus or not? In most cases, however, this difficulty is not very serious, particularly when we are dealing with people old enough to speak. When you go to the oculist he asks you to read a number of letters growing gradually smaller; at some point you fail. Where you have succeeded, he knows that you have perceived enough to make out what letter it is. Or you take a pair of compasses and press the points into a man’s back, asking him if he feels two pricks or only one. He may say one when the two points are near together; if he is on his guard against this error he may say two when in fact there is only one. But if the points are sufficiently far apart he will never make a mistake. That is to say, the bodily movement consisting in pronouncing the word “two” will invariably result from a certain stimulus. (Invariably, I mean, for a given subject on a given day.) This entitles us to say that the man can perceive that there are two points provided they are not too near together. Or you say: “What can you see on the horizon?” One man says, “I see a ship”. Another says, “I see a steamer with two funnels”. A third says, “I see a Cunarder going from Southampton to New York”. How much of what these three people say is to count as perception? They may all three be perfectly right in what they say, and yet we should not concede that a man can “perceive” that the ship is going from Southampton to New York. This, we should say, is inference. But it is by no means easy to draw a line; some things which are, in an important sense, inferential, must be admitted to be perceptions. The man who says “I see a ship” is using inference. Apart from experience, he only sees a queerly shaped dark dot on a blue background. Experience has taught him that that sort of dot “means” a ship; that is to say, he has a conditioned reflex which causes him to utter, aloud or to himself, the word “ship” when his eye is stimulated in a certain way. To disentangle what is due to experience, and what not, in the perceptions of an adult, is a hopeless task. Practically, if a word comes without previous verbal intermediaries, the ordinary man would include what the word means in the perception, while he would not do so if the man arrives at the word after verbal preliminaries, overt or internal. But this is itself a question of familiarity. Show a child a pentagon, and he will have to count the sides to know how many there are; but after a little experience of geometrical figures, the word “pentagon” will arise without any previous words. And in any case such a criterion is theoretically worthless. The whole affair is a matter of degree, and we cannot draw any sharp line between perception and inference. As soon as this is realised, our difficulties are seen to be purely verbal and therefore unimportant.