Do not mistake for happiness the mere absence of pain

This is from my letter to Maria Cosway, October 27, 1786:

Let the gloomy monk, sequestered from the world, seek unsocial pleasures in the bottom of his cell! Let the sublimated philosopher grasp visionary happiness, while pursuing phantoms dressed in the garb of truth! Their supreme wisdom is supreme folly; and they mistake for happiness the mere absence of pain. Had they ever felt the solid pleasure of one generous spasm of the heart, they would exchange for it all the frigid speculations of their lives, which you have been vaunting in such elevated terms.