Cattell on Intelligence:

"Intelligence, or rather “g”, may perhaps be best defined as the ability to perceive relations between things (especially, of course, the ability to appreciate, grasp, and apply the most complicated relationships). It manifests itself in adaptability to new situations (not emotional adaptability), in originality of thought, in the power to reason abstractly, in rapidity of learning, and indeed in almost all the activities of man which are not purely habitual."

Raymond B. Cattell