Maybe it's both? Say, for some potential project, you rate it X on the pessimism/optimism scale. If you're too pessimistic, you won't attempt it, even though it

Maybe it's both? Say, for some potential project, you rate it X on the pessimism/optimism scale. If you're too pessimistic, you won't attempt it, even though it would have worked out nicely! But if you're too optimistic, you'll waste time attempting it even though it has no chance of success. Perhaps the ideal is to be able to correctly assess the chance of success of projects, and then you can make an accurate allocation of effort :)