The doctrine that the interpretation of nature is the interpretation of human experience seems to carry with it the implication that it is built upon purely subjective foundations. Thus Herbert Spencer says: "We can think of matter only in terms of mind;" and Huxley contends that physics must confess that " all the phenomena of nature are known to us only as facts of consciousness." So, too, when we pass, as I propose now to pass, to the superstructure of scientific interpretation, George Henry Lewes assures us that " what we call the Laws of Nature are not objective existences, but subjective abstractions—formulae in which the multitudinous phenomena are stripped of their variety and reduced to unity." In all this there cannot fail to be much that is puzzling to many of us. Hear Huxley again: " Our sensations, our pleasures, our pains, and the relations of these make up the sum total of positive, unquestionable knowledge. We call a large section of these sensations and their relations matter and motion; the rest we term mind and thinking; and experience shows that there is a constant order of succession between some of the former and some of the latter."Does it not seem somewhat repugnant to common-sense to speak of matter and motion, molecules and electrons, as in reality constituted by "a large section of our sensations and their relations"? Can we feel surprise that Mr. Balfour should urge against such a view that "it involves a complete divorce between the practice of science and its theory. It is all very well,"he continues," to say that the scientific account of mental physiology in general, and of sense-perception in particular, requires us to hold that what is immediately experienced are mental facts, and that our knowledge of physical facts is but mediate and inferential. Such a conclusion is quite out of harmony with its own premises, since the proportions on which, as a matter of historical verity, science is ultimately founded are not propositions about states of mind, but about material things." Scientific men, he says a little further on, "have never suspected that while they supposed themselves to be perceiving independent material objects, their qualities and their behaviour, they were in reality perceiving quite another set of things, namely feelings and sensations of a particular kind, grouped in particular ways, and succeeding each other in a particular order".

Now when we consider experience from the plainest and most practical standpoint of common-sense, there is disclosed a duality of reference—an objective reference to things and events independent of us severally and individually, and a subjective reference to our own feelings and emotions and to the stream of our individual thought. This duality of reference is an inalienable feature of our experience, but apart from that experience has no meaning. Instead of saying that a large section of our sensations and their relations are termed matter and motion, while the rest are termed mind and thinking, it would be better to say that the same group of sensations and their relations which constitute our ordinary perceptions exhibit this duality of reference—objective and subjective. When we look out upon a wide and smiling prospect there is a reference both to what we see and what we feel. Each reference belongs to a different universe of discourse, the one objective and the other subjective; and each is a distinguishable aspect of our common experience. We should, therefore, always be prepared to ask and to answer this question with regard to any group of experiences: To which universe of discourse do they belong in the treatment I propose to undertake? If we ask this question with respect to the laws of nature we must reply that so far as physical science is concerned they have unquestionably an objective reference, but that so far as mental science is concerned they have subjective reference in that they have their genesis in the psychological processes of abstraction and generalisation. It is with some of the methods of physical science in their objective reference that we have now to deal.

I said that the manifestation which is interpreted in terms of mechanism is real with all the reality of sensory experience. Let us now turn to this interpretation so as to take note of the method of physical science. We may consider first the apparently simple case of a crowbar employed to raise a heavy mass; and no less distinguished a physicist than Lord Kelvin, in association with the late Professor Tait, shall act as our interpreter. He says that even this simple case is too complex for exact treatment, since its accurate mathematical investigation would involve the discussion of a great number of small movements in every part of the bar, of the fulcrum, and of the mass raised. That is impossible; even this simple case must be simplified. It is a result of observation, however, that the particles retain throughout the process very nearly the same relative positions. Hence the idea of solving, instead of the actual impossible problem, another which is much simpler, but which leads to approximately the same result. Conceive the bar, fulcrum, and mass raised to be perfectly rigid; that is to say, simplify the problem by neglecting all the actual minute changes of position among the particles. Then you can solve the problem ; and the solution will very nearly, but not quite, fit the actual facts of the case. You have substituted an imaginary crowbar, absolutely rigid, for the real crowbar which is not absolutely rigid. You have carried, in a scheme of ideal construction, an observed property of crowbars, that they are comparatively unyielding, to its ultimate limits in thought. There you have in a nutshell the method of physics. It deals with simplified ideal constructions, instead of the complicated actual cases; and it carries its mechanical conceptions to their ultimate limits.

This scientific method of dealing with simplified ideal constructions is exemplified in recent researches on the atom.