The Vectors of Mind presents Thurstone's methods for conducting a factor analysis on a set of variables that allow for more than one factor, an important extension of Spearman's unifactor method. Having multiple factors adds significant complications and much of the book is focussed on the problem of rotation. It attempts to solve this problem by providing an objective basis for the rotation factors, called simple structure, and advocates the use of oblique (correlated) factors to achieve a simple structure. The book utilizes his centroid method of factor extraction, which made it feasible to complete the arduous calculations necessary for a factor analysis at a time when fast electronic computers had not even been imagined. This is a predominantly technical book that relies heavily upon mathematical presentations and provides multiple numerical examples. However, the early chapters delve into philosophical questions of the nature of science and present Thurstone's understanding of measurement theory.
