To complete the perhaps fantastic analogy we must
imagine the world to be one co-ordinated musical system, and our
instrument to be endowed with the power of playing upon the other
keyboards; of thence deriving the suggestion of the distinction between
the internal and external impulses which respectively awakened harmonies
within itself; and lastly, of thus at length conceiving in the spirit of
science that the necessary and universal laws which it recognised as the
most subjective and fundamental conditions of its own operation, at the
same time regulated the activity of the entire musical universe.

How natural it would be for such an intelligent musical instrument, if
unhappily endowed with common sense, to believe and assert that the real
substance of the universe consisted solely of sounds. Yet how evident
would it be to us from our standpoint of more absolute knowledge that
the whole orchestra of sounds, although actual and quite distinct from
consciousness, was still merely phenomenal, and yet withal, in its every
expression, revealed the laws and structure of reality--of the system of
things in themselves--a system the reality of which was dissimilar to
those appearances, though all its laws and structure could be studied
and derived from them.

Berkeley, therefore, erred seriously when he described the idea as a
fainter sensation. Faint subjective reproductions of our sensations, as
of blue, green, or the like, constitute a very insignificant element in
our mental furniture. We seldom pursue so far into detail the ideative
effort. Severely and effectively as Berkeley criticised Locke's account
of abstract ideas, the fact remains that abstraction is a primary
feature of our whole conceptual system; and the abstractable elements of
the sensible presentation being the necessary constituents of all
ideative representation are properly denominated ideal. The one element
of particularity which every idea lacks is the reference to the
transmitted transmutation to which the sensible phenomenon owes its
origin. We derive such reference to the external solely from the
obstructions which our free activity encounters and without which we
could receive no suggestion of the non-ego, and in particular no
suggestion of the dynamic element which fundamentally distinguishes
things from thoughts. The empirical content of experience--the so-called
secondary qualities of bodies--are often called in their subjective
aspect "ideal"