This essay is an attempt to trace Thurstone's thinking from his introduction of multiple factor analysis in 1931 to his successful formulation of a practical centroid solution four years later. The reciprocal influences between Thurstone and Hotelling are especially instructive in this connection. The critical steps played out in the midst of vigorous competition between Spearman, Thurstone, Holzinger, Kelley and Burt to set the future course of factor analysis. The essay concludes with a retrospective appraisal of the contribution of factor analysis to psychology.