g of typewriting we said the stimulus might be
either the copy or ideas. One can write from copy or dictation, in which
the stimulus is the written or spoken word, but one can also write as
one thinks of what one wishes to write. The latter is known as
_centrally initiated action_. That is to say, the stimulus comes from
within, in the brain, rather than from without.

Let us explain this kind of stimulation a little further. Suppose I am
sitting in my chair reading. I finish a chapter and look at my watch. I
notice that it is three o'clock, and recall that I was to meet a friend
at that time. The stimulus in this case is in the brain itself; it is
the nervous activity which corresponds to the idea of meeting my friend.
If we disregard the distinction between mind and body, we may say that
the stimulus for a response may be an idea as well as a perception, the
perception arising from the immediate stimulation of a sense organ, and
the idea arising from an excitation of the brain not caused by an
immediate stimulation of a sense organ.

=Instincts and Habits.= In human action it is evident that there is always
a stimulus to start the nerve-impulse which causes the action. If we
make inquiry concerning the connection between the stimulus and
response; if we ask how it has come about that a particular stimulus
causes a particular response rather than some other possible response,
we find two kinds of causes. In one case the causal connection is
established through heredity; in the other, the causal connection is
established during a person's lifetime through training.

A chicken, for example, hides under some cover the first time it hears
the cry of a hawk; it scratches the first time its feet touch sand or
gravel; it pecks the first time it sees an insect near by. An infant
closes its eyes the first time it feels cold wind blow upon them; it
cries the first time it feels pain; it clasps its fingers together the
first time a touch is felt inside them. The child's nervous system is so
organized that, in each of the cases named, the stimulus brings forth
the particular, definite response. These acts do not have to be
learned.

But it is quite different in typewriting and piano playing. One _must
learn_ what keys on the piano to strike in response to the various
situations of the notes as written in the music. One must also learn the
keys on the typewriter before he can operate a typewriter. And in the
case of other habit