Within recent years the psychologists are doing remarkable work in attacking the problem of the mechanics of mental operations, and already in Europe and America some thirty laboratories have been started for experimental work. The subject is somewhat abstruse for detailed reference here, and it must suffice to say that the psychologist, beginning with observations upon himself, measuring, for example, "the degree of sensibility of his own eye to luminous irritations, or of his own skin to pricking, passes on to like inquiry into the numerical relations between the energy of the stimuli of light, sound, and so forth, and the energy of the sensations which they arouse in the nerve-channels." An excellent summary, with references to the newest authorities on the subject, is given by Prince Kropotkin in the Nineteenth Century of August, 1896. All this, to the superficial onlooker, seems rank materialism. But we cannot think without a brain any more than we can see without eyes, and any inquiry into the operation of the organ of thought must run on the same lines as inquiry into the operations of any other organ of the body. And the inquiry leaves us at the point whence we began in so far as any light is thrown on the connection between the molecular vibrations in nerve-tissue and the mental processes of which they are the indispensable accompaniment. Changes take place in some of the thousands of millions of brain-cells in every thought that we think, and in every emotion that we feel, but the nexus remains an impenetrable mystery. Nevertheless, if we may not say that the brain secretes thought as we say that the liver secretes bile, we may also not say that the mind is detachable from the nervous system, and that it is an entity independent of it. Were it this, not only would it stand outside the ordinary conditions of development, but it would also maintain the equilibrium which a dose of narcotics or of alcohol, or which starvation and gorging alike rapidly upset. In his posthumous essay On the Immortality of the Soul, Hume says: "Matter and spirit are at bottom equally unknown, and we cannot determine what qualities inhere in the one or in the other." That is the conclusion to which the wisest come. And in the ultimate correlation of the physical and psychical lies the hope of arrival at that terminus of unity which was the dream of the ancient Greeks, and to which all inquiry makes approach. How, in these matters, philosophy is at one, is again seen in Huxley's admission that "in respect of the great problems of philosophy, the post-Darwinian generation is, in one sense, exactly where the præ-Darwinian generations were. They remain insoluble. But the present generation has the advantage of being better provided with the means of freeing itself from the tyranny of certain sham solutions."