So far as science gives any positive knowledge as to the relations of mind to matter, it amounts to this: That all we call mind is indissolubly connected with matter through the grey cells of the brain and other nervous ganglia. This is positive. If the skull could be removed without injury to the living organism, a skilful physiologist could play with his finger on the human brain, as on that of a dog, pigeon, or other animal, and by pressure on different notes, as on the keys of a piano, annihilate successively voluntary motion, speech, hearing, sight, and finally will, consciousness, reasoning power, and memory. But beyond this physical science cannot go. It cannot explain how molecular motions of cells of nerve-centres can be transformed into, or can create, the phenomena of mind, any more than it can explain how the atoms and energies to which it has traced up the material universe were themselves created or what they really are. All attempts to further fathom the depths of the unknown follow a different line, that of metaphysics, or, in other words, introspection of mind by mind, and endeavour to explain thought by thinking. On entering into this region we at once find that the solid earth is giving way under our feet, and that we are attempting to fly in an extremely rare atmosphere, if, indeed, we are not idly flapping our wings in an absolute vacuum. Instead of ascertained facts which all recognise, and experiments which conducted under the same conditions always give the same results, we have a dissolving view of theories and intuitions, accepted by some, denied by others, and changing with the changing conditions of the age, and with individual varieties of characters, emotions, and wishes. Thus, mind and soul are with some philosophers identical, with others mind is a product of soul; with some soul is a subtle essence, with others absolutely immaterial; with some it has an individual, with others a universal, existence; by some it is limited to man, by others conceded to the lower animals; by some located in the brain, by others in the heart, blood, pineal gland, or dura mater; with some it is pre-existent and immortal, with others created specially for its own individual organism; and so on _ad infinitum_. The greatest philosophers come mostly to the conclusion that we really know nothing about it. Thus Descartes, after having built up an elaborate metaphysical theory as to a spiritual, indivisible substance independent of the brain and cognisable by self-consciousness alone, ends by honestly confessing ‘that by natural reason we can make many conjectures about the soul, and have flattering hopes, but no assurance.’ Kant also, greatest of metaphysicians in demolishing the fallacies of former theories, when he comes to define his ‘noumenon,’ has to use the vaguest of phrases, such as ‘an indescribable something, safely located out of space and time, as such not subject to the mutabilities of those phenomenal spheres, ...