exceptional frequency, becomes reproduced so easily that eventually
the actual presence of the corresponding external stimuli is no
longer necessary, and it will recur on the vibrations set up by faint
stimuli from within. {70} Sensations arising in this way from
within, as, for example, an idea of whiteness, are not, indeed,
perceived with the full freshness of those raised by the actual
presence of white light without us, but they are of the same kind;
they are feeble repetitions of one and the same material brain
process--of one and the same conscious sensation. Thus the idea of
whiteness arises in our mind as a faint, almost extinct, sensation.

In this way those qualities which are common to many things become
separated, as it were, in our memory from the objects with which they
were originally associated, and attain an independent existence in
our consciousness as IDEAS and CONCEPTIONS, and thus the whole rich
superstructure of our ideas and conceptions is built up from
materials supplied by memory.

On examining more closely, we see plainly that memory is a faculty
not only of our conscious states, but also, and much more so, of our
unconscious ones. I was conscious of this or that yesterday, and am
again conscious of it to-day. Where has it been meanwhile? It does
not remain continuously within my consciousness, nevertheless it
returns after having quitted it. Our ideas tread but for a moment
upon the stage of consciousness, and then go back again behind the
scenes, to make way for others in their place. As the player is only
a king when he is on the stage, so they too exist as ideas so long
only as they are recognised. How do they live when they are off the
stage? For we know that they are living somewhere; give them their
cue and they reappear immediately. They do not exist continuously as