According to R. Cattell, general intelligence comprises two parts: “general fluid factor” (or “free” intelligence) and “general crystallized factor” (or “fixed intelligence”).
“Free intelligence” is mainly formed by biological determinants based on the individual architectonics of cerebral cortex. In this sense, general intellectual ability is being developed at the early stages of individual growth. It does not depend on acquisition of any specific knowledge, skills, or development of some psychical function.
“Fluid” abilities are based on comprehension of the main types of logic correlation and key mental processes: part-whole relationship, understanding of likeness and casual relation, induction, deduction, and special relation. “Fluid” intelligence defines the correlation ability of the objects’ the person is able to apprehend and operate.