THE EXPERIMENTAL work which is now to be described was not planned as a whole. Rather, it grew stage by stage as one inquiry led to another. There are therefore two policies which might be adopted in presenting the results. It would be possible either to report the work largely in historical sequence, or alternatively to arrange it in an order which might appear more logical-for instance, by reporting first of all the results for the youngest subjects and going on then to the fmdings for the older ones. The plan adopted is in effect a compromise. In the various studies that have been conducted five main types of problem have been used. All the evidence relating to anyone type will be drawn together for the purposes of this report - a procedure which will involve departure from historical sequence but will be necessary in order to avoid much repetition. The report on anyone type of problem, however, will in the main follow the order in which the work was carried out, on the ground that this will make possible a clearer presentation of the issues and considerations which have guided the development of the inquiry.