Thomson begins by introducing the concept of individual differences in general terms, before moving on to his own topic of intelligence, and discussing the extent of individual intelligence differences. He illustrates the wideness of the intellectual range within the population by reproducing Cyril Burt's table, Distribution of Intelligence among Children and Adults, which contains suggested IQ scores for different levels of schooling and occupational complexity in England. Thomson gives examples of individuals at the extreme ends of intelligence, referring to precocious children, and to a report by the Mental Deficiency Committee on the prevalence of feeble-mindedness in the general population. He argues that this range is both inevitable and vital for society's progress.

Thomson then talks about the measurement of intelligence and stresses that we cannot fully differentiate human intelligence differences into classes, because in reality no such distinctions exist. Instead, intelligence tests identify an individual's position on a continuum.

Thomson ends with a discussion of the idea of selection, emphasising that it is often wrongly thought of as a process of exclusion, when in fact it should regarded and used as part of the process in establishing the correct path for the student, taking into account both their abilities and their interests. In this process, the educators have a social and professional responsibility to guide them in establishing the correct path for them as individuals. He argues that selection should be based not on a single test but following consideration of the pupils' performance throughout their primary school career.