Mental Energy:

"Productivity implies actual production or objective achievement. For the psychological basis of intellectual productivity in the broadest sense, we need a construct that could be labeled mental energy. This term should not be confused with Spearman's g (for general intelligence). Spearman's theory of psychometric g as "mental energy" is a failed hypothesis and has been supplanted by better explanations of g based on the concept of neural efficiency (Jensen, 1993). The energy construct I have in mind refers to something quite different from cognitive ability. It is more akin to cortical arousal or activation, as if by a stimulant drug, but in this case an endogenous stimulant. Precisely what it consists of is unknown, but it might well involve brain and body chemistry."

Arthur Jensen; 1996
Giftedness and Genius: Crucial Differences