“If two opposite theories are propagated one will be wrong.”

Source: A Treatise On Political Economy (Fourth Edition) (1832), Introduction, p. xxi

On Marxism, which he considered more suited to ants than to humans.
http://arts.independent.co.uk/books/news/article2886394.ece.

"The Fundamentals of Theoretical Physics," (1940) as quoted in Out of My Later Years (1976)
1940s

Industrial Disease
Song lyrics, Love over Gold (1982)

"The Nearest Star" (1989) (reprinted in The Secret of the Universe (1992), p. 82)
General sources

Source: Green Mars (1993), Chapter 8, “Social Engineering” (p. 410)

Notes Toward a Supreme Fiction (1942), It Must Change
Context: p>Two things of opposite natures seem to depend
On one another, as a man depends
On a woman, day on night, the imaginedOn the real. This is the origin of change.
Winter and spring, cold copulars, embrace
And forth the particulars of rapture come.</p

Rumi Daylight (1990)

"Negroes Are Anti-Semitic Because They're Anti-White" http://www.nytimes.com/books/98/03/29/specials/baldwin-antisem.html in The New York Times (9 April 1967)
Context: It is true that two wrongs don't make a right, as we love to point out to the people we have wronged. But one wrong doesn't make a right, either. People who have been wronged will attempt to right the wrong; they would not be people if they didn't. They can rarely afford to be scrupulous about the means they will use. They will use such means as come to hand. Neither, in the main, will they distinguish one oppressor from another, nor see through to the root principle of their oppression.