Never give way to melancholy; resist it steadily, for the habit will encroach.

Sydney Smith (1771–1845) English clergyman, essayist, wit

Memoir of the Reverend Sydney Smith, by His Daughter, Lady Holland, Vol. 1, ch. 10 (1855)
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In chapter 11 is a parallel quotation from Smith: "Never give way to melancholy: nothing encroaches more; I fight against it vigorously."

But Lady Holland observes that in Smith's notebook he also wrote, "I wish I were of a more sanguine temperament; I always anticipate the worst."