Discourse on the Passion of Love, by Blaise Pascal, and essay on The Physical Cause of Love.

“Man is born for thought; therefore he is not a moment without it; but the pure thoughts that would render him happy, if he could always maintain them, weary and oppress him. They make a uniform life to which he cannot adapt himself; he must have excitement and action, that is, it is necessary that he should sometimes be agitated by those passions the deep and vivid sources of which he feels within his heart.

The passions which are the best suited to man and include many others, are love and ambition: they have little connection with each other; nevertheless they are often allied; but they mutually weaken, not to say destroy, each other...”