dy have such a law, which we put to some use in studying
attention, [Footnote: See pp. 268-264.] and called the law of
"combination", or of "unitary response to a plurality of stimuli". We
had better fetch that law out again and put it in good repair, and see
whether it is adequate for the job that we now have on hand. In a very
general, abstract form, the law of combination read that "two or more
stimuli may arouse a single joint response". Let us add a single word,
which had not risen above the horizon when we formulated the law
before, and say that {399} _two or more contiguous stimuli may arouse
a single joint response_.

That seems very little to say; can we possibly go far with so simple a
statement? Well, let us see. In saying that two or more stimuli arouse
a single response, we imply that _there is already some rudimentary
linkage between each stimulus and their common response, and that this
linkage is used in arousing the response_. Now bring in our trusty law
of exercise, and we see that the use, or exercise, of such a linkage
may strengthen it to such an extent that, _later, a single one of the
stimuli may arouse the response which was originally aroused by the
whole collection of stimuli_.

Does that promise any better? Probably it requires further discussion
and exemplification before its value can be appreciated. Let us, then,
first discuss it a bit, and then apply it to the explanation of the
chief varieties of learned reaction that have come to our attention.

The law of combination attempts to show how it comes about that a
stimulus, originally unable to arouse a certain response, acquires the
power of arousing it; and the law states that this occurs only when
the originally ineffective stimulus is combined with others which can
and do arouse the response. The ineffective stimulus, being one of a
combination of stimuli which collectively arouse the response,
participates to some slight degree in arousing that response and may
thus become effectively linked with the response.

Notice an assumption underlying the law of combination. Evidently a
stimulus could not take part in arousing a response unless there were
some pre-existing linkage between it and the response. This linkage
may however be extremely loose and feeble, and wholly incapable by
itself of arousing the response. The assumption of pre-existing loose
linkage between almost any stimulus and almost any response is
justified by the facts