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 habits, we find, for example, that one does not respond by
saying "81" for 9 times 9; nor "13" for 6 plus 7; nor "8" for 15 minus
7; nor "8" for the square root of 64; nor "144" for the square of 12,
etc., until one has learned in each case.

Some connections between stimulus and response we have through
inheritance; all others are built up and established in one's lifetime,
particularly in the first thirty years of one's life.

We have spoken of bonds between stimulus and response, but have not
explained just what can be meant by a _bond_. In what sense are stimulus
and response bound together? A bond is a matter of greater permeability,
of less resistance in one direction through the nervous system than in
other directions. Nerves are conductors for nerve-currents. When a
nerve-current is started in a sense organ, it passes on through the path
of least resistance.

Now, some nerves are so organized and connected through inheritance as
to offer small resistance. This forms a ready-made connection between
stimulus and response. Muscular responses that are connected with their
stimuli through inherited bonds, by inherited nerve structure, are
called instincts. Those that are connected by acquired bonds are called
habits. Sucking, crying, laughing, are instinctive acts. Adding,
typewriting, piano playing, are habits.