iation of ideas. And the great advance of
knowledge which has been made since his day with respect to both these
principles, entitles us to be much more confident than even he was that
they are in some way intimately united. Moreover, the manner in which
they are so united we have begun clearly to understand. For we know from
our study of nerve-action in general, that when once a wave of invisible
or molecular movement passes through any line of nerve-structure, it
leaves behind it a change in the structure such that it is afterwards
more easy for a similar wave, when started from the same point, to
pursue the same course. Or, to adopt a simile from Hobbes, just as water
upon a table flows most readily in the lines which have been wetted by a
previous flow, so the invisible waves of nerve-action pass most readily
in the lines of a previous passage. This is the reason why in any
exercise requiring muscular co-ordination, or dexterity, 'practice makes
perfect:' the nerve-centres concerned learn to perform their work by
frequently repeating it, because in this way the needful lines of
wave-movement in the structure of the nerve-centre are rendered more and
more permeable by use. Now we have seen that in the nerve-centres called
the cerebral hemispheres, wave-movement of this kind is accompanied with
feeling. Changes of consciousness follow step by step these waves of
movement in the brain, and therefore when on two successive occasions
the waves of movement pursue the same pathway in the brain, they are
attended with a succession of the same ideas in the mind. Thus we see
that the tendency of ideas to _re_cur in the same order as that in
which they have previously _oc_curred, is merely an obverse expression
of the fact that lines of wave-movement in the brain become more and
more permeable by use. So it comes that a child can learn its lessons by
frequently repeating them; so it is that all our knowledge is
accumulated; and so it is that all our thinking is conducted.

A wholly new field of inquiry is thus opened up. By using our own
consciousness as a physiological instrument of the greatest delicacy, we
are able to learn a great deal about the dynamics of brain-action
concerning which we should otherwise remain in total ignorance. But the
field of inquiry thus opened up is too large for me to enter upon
to-day. I will therefore merely observe, in general terms, that although
we are still very far from understanding the op