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 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 coöperated 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.