Human beings, we have come to appreciate, are immensely diverse, not only in their physical characteristics, but even more in their behaviors, personalities, and capabilities. It is with a sense of wonderment that the scientist or even the ordinary person on the street contemplates this enormous diversity and hopes to make sense of it by reducing it to some set of cardinal ideas or principles.

In this book, I have focused on just one domain of human diversity – the domain of what may be called cognitive abilities. This domain alone, it appears, is far-flung and finely divided. For more than a century, since the days of those who first broached the possibility of studying individual differences in cognitive abilities – Francis Galton, James McKeen Cattell, Alfred Binet, William Stern, Charles Spearman, and others – psychologists have attempted to find ways of understanding the diversity of these abilities, first by developing procedures for objectively measuring them and then by formulating ways of classifying them and investigating their structure. Two principal instruments for doing these things have been (1) the mental test and (2) the statistical technique known as factor analysis.