“Professor Spearman has drawn over-hasty conclusions”: Unravelling psychometrics’ “Copernican Revolution”

The significance of the dispute between Charles Spearman and Godfrey Thomson over the role of factor analysis in providing unambiguous evidence for Spearman’s two-factor theory has not been recognised for the crucial moment it actually was in the history of psychometrics. Thomson sketched an alternative to Spearman’s theory demonstrating that, factor analysis notwithstanding, there was no need to hypothesise that abilities are quantitative attributes. Spearman was blind to this fact and his stature within the discipline ensured that Thomson’s alternative was subsequently neglected. I argue that Spearman’s blindness was conditioned by his fealty to two idols of the age: the quantitative imperative and the psychometricians’ fallacy, both of which were secured in Spearman’s mind by his failure to fully understand the concept of measurement. Given his influence, Spearman’s blindness to the significance of Thomson’s critique was a cause of psychometrics’ relentless progress to becoming a pathological science.