Ethics, Part IV, Proposition 61

Desire (Def. of the Emotions, i.) considered absolutely is the actual essence of man, in so far as it is conceived as in any way determined to a particular activity by some given modification of itself. Hence desire, which arises from reason, that is (III. iii.), which is engendered in us in so far as we act, is the actual essence or nature of man, in so far as it is conceived as determined to such activities as are adequately conceived through man's essence only (III. Def. ii.). Now, if such desire could be excessive, human nature considered in itself alone would be able to exceed itself, or would be able to do more than it can, a manifest contradiction. Therefore, such desire cannot be excessive. Q.E.D.