[Illustration: Fig. 62.--Learning a word-habit in typewriting. At first, besides the stimulus of the word, "_and_" it is necessary also to have the stimulus "_a_" in order to arouse the response of writing a, the stimulus "_n_" in order to arouse the writing of n, and the stimulus "_d_" in order to arouse the writing of d. Yet the stimulus "_and_" is present all this time, and its weak linkages with the writing movements are used and strengthened, so that finally it is sufficient, by itself, to arouse the whole series of writing movements.] Many other instances of learning can be worked out in the same way, and there seems to be no difficulty in {413} interpreting any of them by the law of combination. Even "negative adaptation" can possibly be interpreted as an instance of substitute response; some slight and easy response may be substituted for the avoiding reaction or the attentive reaction that an unimportant stimulus at first arouses, these reactions being rather a nuisance when they are unnecessary. On the whole, the law of combination seems to fill the bill very well. It explains what the law of exercise left unexplained. It always brings in the law of exercise as an ally, and, in explaining substitute response, it brings in the law of effect, which however, as we saw before, may be a sub-law under the law of exercise. These two, or three laws, taken together, give an adequate analysis of the whole process of learning. The Law of Combination in Recall Unitary response to multiple stimuli is important in recall as well as in learning. The clearest case of this is afforded by "controlled association". [Footnote: See p. 381.] In an opposites test, the response to the stimulus word "long" is aroused partly by this stimulus word, and partly by the "mental set" for opposites. There are two lines of influence, converging upon the response, "long--short" (of which only the word "short" may be spoken): one line from the stimulus word "long", and the other from the mental set for pairs of opposite words. The mental set for opposites tends to arouse any pair of opposites; the word "long" tends to arouse any previously observed group of words of which "long" is a part. The mental set, an internal stimulus, and the stimulus word coming from outside, converge or combine to arouse one particular response. The mental set for adding has previously exercised {414} linkages with the responses composing the addition table, while the mental set for multiplication has linkages with the responses composing the multiplication table. When the set for adding is active, a pair of numbers, seen or heard, together with this internal stimulus of the mental set, arouses the response that gives the sum; but when the multiplying set is active, the same pair of numbers gives the product as the response. All thinking towards any goal is a similar instance of the law of combination. The Laws of Learning in Terms of the Neurone