Knowledge and Passion
Whatever moralists may hold, the human understanding is greatly indebted to the passions, which, it is universally allowed, are also much indebted to the understanding. It is by the activity of the passions that our reason is improved; for we desire knowledge only because we wish to enjoy; and it is impossible to conceive any reason why a person who has neither fears nor desires should give himself the trouble of reasoning. The passions, again, originate in our wants, and their progress depends on that of our knowledge; for we cannot desire or fear anything, except from the idea we have of it, or from the simple impulse of nature.
Jean-Jacques Rousseau, in 'Discourse on the Origin of Inequality'