“It is impossible to do anything for anyone.”

As quoted in "The Vindication of Edwin Land" in Forbes magazine, Vol. 139 (4 May 1987) p. 83; this was later humorously altered:
Don't do anything that someone else can do. Don't undertake a project unless it is manifestly important and nearly impossible.
Edwin Ladd, Department of Astronomy, Harvard University (August, 1990), as quoted by Lincoln J. Greenhill at Harvard University http://www.cfa.harvard.edu/~lincoln/

Source: The Meditations of the Emperor Marcus Aurelius Antoninus

Source: Political Treatise (1677), Ch. 2, Of Natural Right
Context: In the state of nature, wrong-doing is impossible; or, if anyone does wrong, it is to himself, not to another. For no one by the law of nature is bound to please another, unless he chooses, nor to hold anything to be good or evil, but what he himself, according to his own temperament, pronounces to be so; and, to speak generally, nothing is forbidden by the law of nature, except what is beyond everyone's power.<!-- 23

Illusions : The Adventures of a Reluctant Messiah (1977)

Quote in Bazille's letter to his father, 1864; as cited in: article: Frédéric Bazille and the Birth of Impressionism, Corrinne Chong, PhD -independent scholar http://www.19thc-artworldwide.org/autumn17/chong-reviews-frederic-bazille-and-the-birth-of-impressionism
1861 - 1865

Source: Thinking and Destiny (1946), Ch. 14, Thinking: The Way to Conscious Immortality, p. 943