upon the surface of the retina, which is a singularly delicate apparatus connected with the terminations of the fibres of the optic nerve.  The impulses of the attenuated matter, or ether, affect this apparatus and the fibres of the optic nerve in a certain way; and the change in the fibres of the optic nerve produces yet other changes in the brain; and these, in some fashion unknown to us, give rise to the feeling, or consciousness, of redness.  If the marble could remain unchanged, and either the vibrations of the ether, or the nature of the retina, could be altered, the marble would seem not red, but some other colour.  There are many people who are what are called colour-blind, being unable to distinguish one colour from another.  Such an one might declare our marble to be green; and he would be quite as right in saying that it is green as we are in declaring it to be red.  But then, as the marble itself cannot be both green and red, at the same time, this shews that the quality redness must be in our consciousness and not in the marble.”

In similar fashion he shewed that the hardness, roundness, and even the singleness of the marble were, so far as we know, states of our consciousness and not in the marble.  The argument is capable of application to all that we call matter, and it thus appears, on analysis, that what we know of matter is simply a series of states of our consciousness, or mind.  In similar fashion, it turns out that what we call mind is, so far as practical experience goes, always associated with and dependent on what we call matter.  We have no direct knowledge of thinking without a brain, or of consciousness without a body.  Alterations and changes in matter, as for instance in the tissues and nutrition of the body, are, so far as our experience goes, inseparably associated with mental operations.  In the animal kingdom we see the development of the mind creeping slowly after the development of the material nervous system, until, in man, the most complex mind and most complex consciousness of which we have knowledge accompany the most complex body and brain.

Two great rival solutions to this fundamental problem are Materialism and Idealism.  Materialism supposes that what we call matter is the real substance of the universe, and that mind is merely one of the forms of its activity.  The advance of physical science has done much to make the materialistic hypothesis more plausible.  When matter was believed to be inert, the mere vehicle or theatre of forces, materialism remained a singularly crude and unsatisfying position.  But now that science has shewn all that we call matter—­the most solid metals and the most attenuated vapours, the most stable and resisting inorganic bodies, and the unstable tissues of living bodies—­to be alike in restless, orderly motion, to be, in fact, motion itself and not the thing moved, to be changeable but indestructible, passing through phases but eternal, there seems less difficulty in assuming it to be the ultimate reality, and mind and consciousness to be its most highly specialised qualities.  Huxley, while stating this view plainly enough, refused to accept it as a legitimate conclusion from the facts.