“He has no native Passion, because he is not a Thinker — & has probably weakened his Intellect by the haunting Fear of becoming extravagant.”

Daedalus or Science and the Future (1923)

Source: The Unbearable Lightness of Being

1830s, The American Scholar http://www.emersoncentral.com/amscholar.htm (1837)

Daedalus or Science and the Future (1923)
Variant: The conservative has little to fear from the man whose reason is the servant of his passions, but let him beware of him in whom reason has become the greatest and most terrible of passions. These are the wreckers of outworn empires.

ibid

19 December 1749
Letters to His Son on the Art of Becoming a Man of the World and a Gentleman (1774)
Context: We must not suppose that, because a man is a rational animal, he will, therefore, always act rationally; or, because he has such or such a predominant passion, that he will act invariably and consequentially in pursuit of it. No, we are complicated machines; and though we have one main spring that gives motion to the whole, we have an infinity of little wheels, which, in their turns, retard, precipitate, and sometime stop that motion.

On the Norman conquest of England; Vol I; The Birth of Britain.
A History of the English-Speaking Peoples (1956–58)