Thurstone proposed a group factor theory of intelligence that defined it as consisting of 7 primary mental abilities: verbal comprehension, verbal fluency, numerical ability, perceptual speed, inductive reasoning, spatial visualization, and memory. He later added 2 more factors: deductive reasoning and problem solving ability. Each factor is relatively independent and involves different mental operations or skills. Thurstone's theory contributed to the measurement of intelligence and attitudes and influenced later theories of multiple intelligences. However, its limitation was discarding the concept of a common general intelligence factor.