For the practical purposes of daily life, guidance is afforded by the correlation of the several fields of sensory reference the visual field, the auditory field, the fields of touch, of smell, taste, temperature and so forth; and the field of motor activity, that which includes all those sensory impressions which afford information of the movements of our limbs and bodies. Through this correlation experience becomes an organised whole, and the data from the several fields are brought to bear on the conduct of life. Presentations of all sorts get their value in relation to our practical needs. And for these ends all are equally real and valid as given in experience. But when our aim is the interpretation of nature, as the objective reference of all such experience, it has been found convenient to regroup the data for the purposes of scientific study. Thus we have the facts of experience which fall under astronomy, geology, chemistry, physics, and so forth. And with this regrouping is associated a detachment from the immediate purposes of practical life at any rate so far as concerns what is spoken of as pure science the aim of applied science being a re-attachment of the results of the inquiries of specialists to our industrial needs and requirements. But, in all the sciences above named, matter in motion plays a part; while in all mental science physiological changes, themselves susceptible of interpretation in terms of matter and motion, are at all events accompanying phenomena. Since this is so (and such is the claim of naturalism), all these modes of experience are directly or indirectly interpretable in terms of matter and motion in terms of mechanism. Now where exact mathematical treatment is desirable it is obviously convenient—nay, more, it may be necessary to translate divers kinds of experience into the terms of that category which is most completely susceptible of such treatment. That is what the physicist, as such, does. He says in effect: Here are a number of categories of experience; a, b, c, d, e; my business is to interpret nature in terms of physical mechanism c; I, therefore, for the purposes of my special type of interpretation, translate a and b (let us say colour and sound phenomena) into terms of c, in which terms I can treat them mechanically and mathematically. This is surely a quite justifiable methodological procedure. I take it that the physicist, as mere man, never dreams of saying that the colour of a ripe apple or the notes of a woodlark's song are, as modes of experience, less real for the purposes of daily life than the ethereal undulations or the auditory vibrations into which he translates them for the purposes of physical explanation. He hears, like one of us, the Kreutzer Sonata, or sees, in the Dresden gallery, the Sistine Madonna. These modes of experience are just as real and valid for him as they are for you and me. But he may urge that these art-products are closely attached to human interests, while, in his laboratory, he studies, in and for themselves, the sound or colour elements out of which these products are compounded. And he may further urge that in his physical investigations he is disclosing realties of a different and more permanent order than those of music and painting. Such a contention, supposing it to be urged by the physicist, as it is by the exponent of naturalism, opens up a broad philosophical question as to the nature of experience. John Locke faced the question, and his conclusions are well known. He divided the properties of objects into two groups. Of these the so called secondary qualities of colour, sound, odour, temperature, and the like depend on the constitution of the human or other being who has experience of them; but the so called primary qualities, extension and motion, have an independent existence apart from all human or other experience. Of the former he says: " Take away the sensation of them; let not the eye see light or colour, nor the ear hear sounds; let the palate not taste, nor the nose smell; and all colours, tastes, odours, and sounds, as they are such particular ideas, vanish and cease, and are reduced to their causes i.e. bulk, figure, and motion of parts".
But a few years after Locke's death the youthful and brilliant George Berkeley attacked this position, and proved by the most convincing logic that it is untenable. He showed conclusively that it is quite impossible to regard the primary qualities as any more independent of human experience than those which had been termed secondary. Their nature, in so far as known to us, is every whit as much in relation to certain modes and combinations of human experience. The effect of his now familiar argument was reconstructive as well as destructive. It was reconstructive in that it rehabilitated the categories of colour, sound, odour, and so forth as belonging to the same order of reality, in and for experience, as extension and motion. It was destructive in that it dealt the death blow to a belief in the independent existence of matter and motion as such apart from experience. It knocked the bottom out of materialism as a philosophy. And no amount of soldering and tinkering can make it again hold water.
Let us, however, look a little further into the question at issue between Locke and Berkeley.