Courage is the principal virtue, for all the others presuppose it. If you are afraid, you may do anything.

Robert Louis Stevenson (1850–1894) Scottish essayist, novelist, poet

Essay (1880-01/02?), “Reflections and Remarks on Human Life,” § 12 “Courage”
    (Source)

A collection of aphorisms and musings, first published in the Edinburgh Edition of his Works, vol. 28 (1898).

See Johnson (1775), Barrie (1922), Lewis (1942).