I do not mean to attempt a summary of psychology from this point of view, any more than I have attempted a summary of physics or physiology. My argument is really independent, in this case as in the other, of particular systems. All I ask for is the admission that in perception there are conditions antecedently supplied by the perceiving consciousness which profoundly modify every perceptual experience—and that these conditions (unlike Kant’s forms) are natural growths, varying, like other natural growths, from individual to individual. This admission must, I think, be made by every empirical psychologist, to whatever school he happens to belong.
If this statement seems obscure in its general and abstract form, consider a particular application of it. Let us assume, with many psychologists, that Will, in the form of selective attention, lies at the root of our perceptual activities; that we may therefore be said, in a sense, voluntarily to create the objects we perceive; that experience of the present is largely qualified by memories of the past, and that the perceptual mould into which our sensations are run is largely a social product—born of the intercourse between human beings, and, in its turn, rendering that intercourse possible. Is it not clear that, on assumptions like these, consciousness, so far from passively receiving the messages conveyed to it through physical and physiological channels, actively modifies their character?
V
But why, it may be asked, should these considerations involve any difficulty? And, if there be a difficulty, what is its exact character?
In its most general form the difficulty is this. It is claimed by science that its conclusions are based upon experience. The experience spoken of is unquestionably the familiar perception of external things and their movements as understood by common sense; and, however much our powers of perception be increased by telescopes, microscopes, balances, thermometers, electroscopes, and so forth, this common-sense view suffers no alteration. The perceptions of a man of science are, in essence, the perceptions of ordinary men in their ordinary moments, beset with the same difficulties, accepted with the same assurance. Whatever be the proper way of describing scientific results, the experimental data on which they rest are sought and obtained in the spirit of “naïf realism.”
On this foundation science proceeds to build up a theory of nature by which the foundation itself is shattered. It saws off the branch on which it is supported. It kicks down the ladder by which it has climbed. It dissolves the thing perceived into a remote reality which is neither perceived nor perceivable. It turns the world of common sense into an illusion, and on this illusion it calmly rests its case.
But this is not the only logical embarrassment in which we are involved. When science has supplied us with a description of external things as they “really are,” and we proceed to ask how the physical reality reveals itself to us in experience, a new difficulty arises, or, if you like, the old difficulty with a new face. For science requires us to admit that experience, from this point of view, is equivalent to perception; and that perception is a remote psychological effect of a long train of causes, physical and physiological, originally set in motion by the external thing, but in no way resembling it. Look carefully at this process from the outside, and ask yourselves why there should be any such correspondence between the first of these causes and the last of these effects, as should enable us to know or infer the one from the other? Why should the long train of unperceivable intermediaries that connect the perceived with the perceiver be trusted to speak the truth?
I just now likened these intermediaries to relays of messengers. But messengers are expected to hand on their message in the form in which they have received it. The messengers change, but not the message. The metaphor, therefore, is far too complimentary to the train of physical causes which reveal the material thing to the perceiving consciousness. The neural changes which are in immediate causal contiguity with that psychical effect which we call “the experience of an external object” have no resemblance whatever either to the thing as it is perceived or to the thing as it really is. Nor have they any resemblance to the proximate cause which sets them going, namely, the ethereal vibrations; nor have these to the accelerated electrons which constitute the incandescent object which we “experience” as the sun. Nor has the sun, as experienced, the slightest resemblance to the sun as it really is.
Hume, in his “Dialogues on Natural Religion,” urges the absurdity of arguing from an effect like the universe to a cause like God, since the argument from a particular effect to a particular cause, or from a particular cause to a particular effect, is only legitimate when we have had some previous experience of that particular class of causal sequence; and nobody, it is plain, has had the opportunity of observing Creation. Whatever be the value of this argument in the case of God and the world, it seems to me conclusive in the case of matter and man. We cannot argue from purely psychical effects, like perceptions and sensations, to external causes, like physiological processes or ethereal vibrations, unless we can experience both sets of facts in causal relation. And this, if we accept the conclusions of science, we can never do—partly because the intermediate members of the causal series are unperceivable; partly because, if they were perceivable, perception has been reduced by science to a purely psychical effect—which obviously cannot include its material cause. This last must for ever remain outside the closed circle of sensible experiences.
Here, of course, we find ourselves face to face with a familiar objection to those philosophies of perception which deny that we have any access to external reality, except through ideas which are its copy. But they are in a better case than science. They need not explicitly admit a discrepancy between their premises and their conclusions. They arrive at the subjectivity of perception by methods of introspection. They interrogate consciousness, and are convinced that every experience can be analysed into sensations and ideas, some of which, no doubt, suggest externality, but none of which are external. If, then, the worst comes to the worst, they can, and often do, lighten their philosophic ship by pitching the whole material universe overboard as a bit of superfluous cargo. But physical science cannot (at least in my opinion) do anything of the kind. Its whole business is with the material universe. Its premises are experiences of external things, not of internal sensation and ideas. And if it has associated its fortunes with a theory of perception which treats experience as a natural effect of the thing experienced; if it has thereby wandered within sight of the perilous problems which haunt the frontier where mind and matter meet, it has not done so in a spirit of reckless adventure, but in the legitimate pursuit of its own affairs.