Conversation so far:
What is the relation between Raymond and Factor analysis?
In the 1920s, Cattell worked with Charles Spearman who was developing the new statistical technique of factor analysis in his effort.
What is the purpose of the effort applied?
Factor analysis became a powerful tool to help uncover the basic dimensions underlying a confusing array of surface variables within a particular domain.
How do they come about with the factor analysis?
Factor analysis was built upon the earlier development of the correlation coefficient, which provides a numerical estimate of the degree to which variables are "co-related".

Question: What is the meaning of correlated variables?
Self-contained rewrite: What is the meaning of correlated variables?