It is one of the most beautiful compensations of this life that no man can sincerely try to help another without helping himself.

Charles Dudley Warner (1829–1900) American essayist and novelist

Backlog Studies, Fifth Study, sec. 3 (1872)
    (Source)

Originally published in Scribner's Monthly (Apr 1872). Frequently misattributed to Ralph Waldo Emerson.