of playful behavior and trial and error {400}
behavior. In addition to the close reflex connections provided in the
native constitution, and in addition also to the close connections
formed in previous training, there are at any time, and especially in
childhood and youth, a vast number of loose connections. These are too
weak to operate singly, until they have cooeperated in producing a
response, and thus been individually strengthened, after which they
may be able singly to produce the response.

The law of combination, then, as applied to learning, includes four points:

(a) A collection of stimuli may work together and arouse a single
response.

(b) This is possible because of pre-existing loose linkage between
the separate stimuli and the response.

(c) When any stimulus, working together with others, helps to arouse
a response, its linkage with that response is strengthened by
exercise.

(d) The linkage may be sufficiently strengthened so that a single
stimulus can arouse the response without help from the other
stimuli that were originally necessary.

Having now abundantly stated and reiterated the law of combination in
the abstract, let us turn to concrete instances of learned reactions,
and see how the law takes care of them. We have already classified a
large share of all the concrete instances under a few main heads, as
substitute stimulus, substitute response, combination (or association)
of stimuli, and combination of responses. We shall presently find it
possible to reduce these four classes to two, since the association of
two objects, by virtue of which one of them later recalls the other,
is a rather complicated case of substitute stimulus, while the
combination of movements into a higher unit is a complicated case of
substitute response.

[Footnote: To distinguish between "substitute stimulus" and
"substitute response" is, in strict logic, like distinguishing
between "inside out" and "outside in." Whenever there is a
substitute stimulus there is also a substitute response, of course,
since this stimulus, in being substituted for another, gets that
other's response in place of its own original response; and in the
same way, you can always find substitute stimulus in any instance of
substitute response; for, in being substituted for another, a
response gets that other's stimulus in place of its own original
stimulus. For all that, t