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Charles Spearman was an English Psychologist who studied human intelligence. He developed the concept of factor analysis, which is he used to study mental aptitude tests. He found that people who did well on one aptitude measure tended to do well on others and conversely, people who did poorly on one aptitude measure tended to do poorly on most others.
This basic concept of correlation of ability is the basic concept of how Spearman sees intelligence, something that he called the "g" factor, which stands for general intelligence. Basically, although the concept is very complicated, and involves numerous mathematically correlations, the essential part of the theory is that intelligence in one area is typically positively correlated with intelligence in other areas. Therefore, if a person shows an intellectual ability in one specific area, then they will show the same ability across the board.
Because of Spearman's mathematical orientation and his background with statistics, he felt that human intelligence could be expressed in mathematical terms, using a combined number of traits. This is similar to modern trait and factor views as well as the mathematical formulas for intelligence first put forth by Lewis Terman. Terman created the model for mental age divided by chronological age and gave us one of the first mathematical concepts for measuring the mathematical construct known as I.Q.
Spearman's theory was able to take numerous variables and correlated into one expressed value. This paved the way for future testing of intelligence and represents the basic concept of how we view intelligence today.
I hope this has helped,