Instead of speaking of different orders of reality, we should rather speak of the co-ordinate realities on different planes of the analysis and synthesis of objective experience. Instead of saying that a rosebud is not coloured, but capable of reflecting ethereal undulations, we ought to say that it is both coloured and is a distributor of ether-waves; that these are but two ways of dealing with experience at different planes of interpretation—the one in terms of daily perception, the other in terms of physical science; and that both expressions are dealing with the same order of reality, just in so far as they are dealing with what experience does actually disclose. Recent theories of atomic structure have opened up a new plane of analysis. But to say that these results render our ordinary perceptual experience in any sense illusory or mendacious is unsatisfactory, since it tends to break the solidarity of objective knowledge. Once admit freely and fully the co-ordinate value of all the varied forms of human experience, and the interpretation of nature becomes one closely-related, intelligible whole, to which every possible mode and shade of experience contributes in rendering a connected and rational account of the world in which we live.
But naturalism, according to Mr. Balfour, is deeply committed to the distinction between the primary and the secondary qualities of matter. On the other hand, I said in the last section that Berkeley had conclusively shown that this distinction could not be maintained, since the primary qualities, in so far as known to us, are every whit as much in relation to certain modes and combinations of human experience. Apart from perception and conception, resistance, extension, and the rest have, as such, no being of which we can have any cognisance. There is, however, a sense in which it may be said that colour, odour, sound, warmth, and cold belong to different categories of experience from those with which physical science deals in its interpretation of nature. We cannot explain colour phenomena in terms of cold, nor sound phenomena in terms of smell; nor, for the matter of that, can we interpret warmth in terms of resistance or musical timbre in terms of extension. But on a deeper plane of analysis we can render explanations of all the common forms of experience in terms of molar, molecular, atomic, or subatomic motion. Nor can we, indeed, apart from some spiritualistic hypothesis which takes us beyond the range of physical science, conceive of any mode of sensory experience which would not yield on physical analysis factors belonging to one of these categories. Hence these are for science universal in a sense that colour phenomena, sounds, odours, and so forth, are not; and therefore for physical science, just because it is physical science and deals with phenomena on this plane of analysis, all explanations involve translation into terms of appropriate reference. But to admit, nay, contend, that these terms are realities for physical thought is not to allow that any of the modes of human experience are necessarily illusory and mendacious.
It is unquestionably true that the concepts of physical science in terms of atomic and subatomic mechanism involve the process of ideal construction to be presently considered; but this does not remove them into a different order of reality, certainly not into an order of reality independent of human experience. It is just their validity for this experience which brings them within the scope of the only order of objective and physical reality of which we have any cognisance.
That we are incapable of attaining to any knowledge of material existence save as it takes form in relation to perception and conception was part of Berkeley's imperishable contribution to philosophic thought. It is true that the phraseology he employed rendered him liable to misconstruction. He used the word "idea" not only for all that is represented in mental imagery, but for all that is directly presented through the channels of sensation. The sensory impression of a rose-bud actually seen is, in Berkeley's terminology, an idea. But he never dreamt of denying the reality of common experience, and he would have been the last to admit that it was illusory. "I do not," he said, "argue against the existence of any one thing that we can apprehend, either by sense or reflection. That the things I see with mine eyes and touch with my hands do exist, really exist, I make not the least question." And again, "If by matter you understand that which is seen, felt, tasted, and touched, then I say matter exists; I am as firm a believer in its existence as anyone can be, and herein I agree with the vulgar."And it may be confidently asserted that his arguments leave scientific results within the field of experience absolutely untouched. He was not attacking science or common sense, but an early and crude form of naturalism. One may accept his doctrine of experience without for one moment denying either the reality for practical life of the familiar objects around us or the immense value for science of physical explanation in terms of mechanism; terms in which they are susceptible of rigid mathematical treatment. Only we must remember that this mechanism is a manifestation to human experience. But a manifestation of what? Unless we are prepared to assert that our experience of the world is self-originating (whatever that may mean), we must postulate an existence which gives rise to the presentations of sense out of which that experience is elaborated. And this existence is independent of us in whom the experience may or may not be generated. Can we know anything of its attributes as thus independent? Notwithstanding all that has been written by philosophers concerning the Absolute, the Unconditioned, and so forth, I question whether unaided thought and reason can ever disclose the positive attributes of this existence, as it is out of all relation to human experience. To do so we must somehow get outside our own experience; and that is a feat which we cannot compass. Can we then gain no further insight into the attributes of reality than is afforded by the surface experience of presentations? I venture to think that we can, so far as it is related to us in other ways. At the present stage of my argument, however, I can only put my thought in the form of a question: May not the reality which lies behind the phenomena of this universe be a purpose of which they are the expression in relation to human purposes? And may not this be a deeper and more spiritual revelation of the nature of existence than the manifestation in terms of mechanism? That manifestation is real with all the reality of sensory experience and scientific thought. But may not the revelation of purpose be equally real though it is in touch with another order of experience? For the present I must be content to leave this in the form of a suggestive question.