From the time Spearman discovered g in 1904, people have been skeptical about the idea that intelligence was one entity in the mind that could be summarized by a single number. In the Introduction, I showed how psychologists in the twentieth century used factor analysis to argue about whether intelligence was one entity (as Spearman believed) or consisted of multiple mental abilities (as Thurstone claimed). For decades, psychologists repeatedly gathered data, performed factor analyses, and modified their tests, statistical methods, and theories in an effort to better understand intelligence. Though it was a slow process that lasted over half a century, it was productive in shedding light on the debate over the nature of intelligence.