veral times spoken of as being
observed would here be included under "things"; but the strict logic
of the whole classification is not a matter of importance, as the only
object in view is to call attention to the great variety of facts that
are perceived.

Now the question arises, by what signs or indications these various
facts are perceived. Often, as we have seen, the fact is by no means
fully presented to the senses, and often it is far from easy for the
perceiver to tell on what signs the perception depends. He knows the
fact, but how he knows it he cannot tell. A large part of the very
extensive experimental investigation of perception has been concerned
with this problem of ferreting out the signs on which the various
perceptions are based, the precise stimuli to which the perceptions
respond.

For example, we can examine objects by feeling of them with a stick
held in the hand, and thus perceive their roughness or smoothness; but
how do we sense these facts? It seems to us as if we felt them with
the end of the stick, but that is absurd, since there are no sense
organs in the stick. It must be that we perceive the roughness by
means of sensations arising in the hand and arm, but to identify these
sensations is a much harder task than to discover the objective fact
of roughness.

Again, we distinguish the tones of two musical instruments by aid of
their overtones, but elaborate experiments were required to prove
this, since ordinarily we do not distinguish the overtones, and could
simply say that the instruments sounded differently, and let it go at
that.

Once more, consider our ability to perceive time intervals; {439} and
to distinguish an interval of a second from one of a second and a
quarter. How in the world can any one perceive time? Time is no force
that could conceivably act as a stimulus to a sense organ. It must be
some change or process that is the stimulus and that serves as the
indication of duration. Most likely, it is some muscular or internal
bodily change, but none of the more precise suggestions that have been
offered square with all the facts. It cannot be the movements of
breathing that give us our perception of time, for we can hold our
breath and still distinguish one short interval from another. It
cannot be the heart beat, for we can beat time in a rhythm that cuts
across the rate of the heart beat. When a singer is accompanying
himself on the piano, keeping good time in spite of