Well, originally mental energy was thought by Freud to have a direct
neurophysiological correlate. Nowadays it's generally used as an
extensional referent to certain feelings; my guess is that the feelings
are the phenomenal aspect of the "value" slots of our goals. High value
- high mental energy, go out and do it; low value - low mental energy,
just stay home.
Of course mental energy has many origins, which is to say that there are
a whole host of things which affect the "value" slots of goals (be
careful, because what we perceive as the value of a goal is generally
its "justification" slot). Caffeine, for one. In any case, I wish to
avoid a fruitless riffling-through of all the things that other people
think give rise to the feeling of mental energy, and simply state that
my hypothesis assumes these other things to cause, say, an alteration in
brain chemistry (high serotonin levels?) which raises the effective
level of the "value" slot, which gives rise to a phenomenal experience
of mental energy.
--
sentience@pobox.com Eliezer S. Yudkowsky
http://tezcat.com/~eliezer/singularity.html
http://tezcat.com/~eliezer/algernon.html
Disclaimer: Unless otherwise specified, I'm not telling you
everything I think I know.