[Illustration: Fig. 58.--Response by analogy. The letters, A, B, X, Y, represent the several stimuli that make up the original object, and each of them becomes well linked with their common response (seeing the object, and perhaps naming it). When the linkage between X and Y and the response has become strong, a similar object, presenting X and Y along with other new stimuli, C and D, appears, and arouses the old response, by virtue of the now-effective linkage from X and Y to this response.] [Illustration: Fig. 59.--Association by similarity. Everything here as in the previous diagram, except that C and D get a response in addition to that aroused by X and Y, and so the new object is seen to be new, while at the same time it recalls the old object to mind.] The machinery of response by analogy is easily understood by aid of the law of combination. A complex object, presenting a number of parts and characteristics, arouses the response of seeing and perhaps naming the object. This is a unitary response to a collection of stimuli, and each of the parts or characteristics of the object participates in arousing the response, and the linkage of each part with the response is thus strengthened. Later, therefore, the whole identical object is not required to arouse this same {408} response, but some of its parts or characteristics will give the response, and they may do this even when they are present in an object that has other and unfamiliar parts and characteristics. The machinery of association by similarity is the same, with the addition of a second response, called out by the new characteristics of the present object. II. SUBSTITUTE RESPONSE EXPLAINED BY THE LAW OF COMBINATION The substitute response machinery is more complicated than that of the substitute stimulus, as it includes the latter and something more. What that something more is will be clear if we ask ourselves why a substitute response should ever be made. Evidently because there is something wrong with the original response; if that were entirely satisfactory, it would continue to be made, and there would be no room for a substitute. The original response being unsatisfactory to the individual, how is he to find a substitute? Only by finding some stimulus that will arouse it. This is where trial and error come in, consisting in a search for some extra stimulus that shall give a satisfactory response. Suppose now that the extra stimulus has been found which arouses a satisfactory substitute response. The original stimulus, or the reaction-tendency aroused by it, still continuing, participates in arousing the substitute response, playing the part of the originally ineffective stimulus in the conditioned reflex. Thus the original stimulus becomes strongly linked with the substitute response.