WE HAVE JUST SEEN how the subject goes about separating out factors in order to determine their respective effects in a multifactor experimental setup. The present chapter takes up the reactions of the child and adolescent in an analogous situation 2 with the difference that only one of the possible factors actually plays a causal role; since the others have no effect they must be excluded after they have been isolated. Such is the case for the pendulum. The variables which, on seeing the apparatus, one might think to be relevant are: the length of the string, the weight of the object fastened to the string, the height of the dropping point ( = amplitude of the oscillation), and the force of the push given by the subject. Since only the first of these factors is actually relevant, the problem is to isolate it from the other three and to

exclude them. Only in this way can the subject explain and vary the frequency of oscillations and solve the problem.