The investment theory:

"Briefly, the investment theory is that gf is a generalized inherent capacity to perceive relations, based on total volume of effective cortical cells. In the course of school and life experience this potential enables the individual to perceive and commit to memory all sorts of relations he perceives in the real world. One can think of gf as describing the power of a process and gc as being the product resulting from gf and experience."

Intelligence, 1987  - Raymond B. Cattell