J.P. Guilford (1897-1987) was an American psychologist who was one of the first supporters of the use of factor analysis in the field of personality assessment. He used psychometrics to study areas such as intelligence and creativity in individuals. Psychometrics is the area of psychology that focuses on testing things like IQ, aptitudes, and personality traits using psychometric tools such as standardized tests, inventories, and questionnaires.
Guilford viewed intelligence as multifaceted and based on many different variables. He did not agree with theories that suggested that one intelligence measurement was capable of accurately and reliably measuring ALL individuals. He believed that bias in testing and differing methods of problem solving and reaching conclusions made it impossible for a single one factor concept of intelligence to be viable. He developed the Structure of Intellect (SI) theory in which intelligence is comprised of 150 differing mental capabilities that are categorized within three dimensions: contents (such as visual, auditory, behavioral), products (such as relations, classes, systems), and operations (such as memory, cognition, evaluation).