urface 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