This chapter introduces the application of factor analysis to the study of primary mental abilities, as described in L. L. Thurstone's *The Vectors of Mind*. The study, which has been ongoing for several years, aims to identify and isolate the fundamental abilities that underlie various cognitive tasks. Thurstone and his team postulated several tentative psychological categories or factors, which were then tested using a battery of 56 psychological tests. The results revealed distinct primary factors, such as verbal, numerical, and spatial reasoning, but also showed that some factors were more complex and did not fit neatly into the initial categories. For example, two distinct verbal factors were identified, and the number factor was found to be highly restricted. The chapter discusses the methodological strengths of factor analysis, noting that it provides a more precise and objective approach to understanding psychological phenomena compared to traditional classification methods. It also highlights the need for further experimental studies to refine and validate the identified factors, particularly in the areas of perceptual and inductive reasoning. The chapter concludes by outlining the structure of the test battery and the administration process, emphasizing the importance of a diverse and comprehensive set of tests to capture a wide range of cognitive abilities.This chapter introduces the application of factor analysis to the study of primary mental abilities, as described in L. L. Thurstone's *The Vectors of Mind*. The study, which has been ongoing for several years, aims to identify and isolate the fundamental abilities that underlie various cognitive tasks. Thurstone and his team postulated several tentative psychological categories or factors, which were then tested using a battery of 56 psychological tests. The results revealed distinct primary factors, such as verbal, numerical, and spatial reasoning, but also showed that some factors were more complex and did not fit neatly into the initial categories. For example, two distinct verbal factors were identified, and the number factor was found to be highly restricted. The chapter discusses the methodological strengths of factor analysis, noting that it provides a more precise and objective approach to understanding psychological phenomena compared to traditional classification methods. It also highlights the need for further experimental studies to refine and validate the identified factors, particularly in the areas of perceptual and inductive reasoning. The chapter concludes by outlining the structure of the test battery and the administration process, emphasizing the importance of a diverse and comprehensive set of tests to capture a wide range of cognitive abilities.