“A condition which of declension would indicate a devil, may of growth indicate a saint.”

Speech to the American Bar Association (2 September 1925).
Context: The most ominous spirit of our times, as it seems to me, is the indication of the growth of an intolerent spirit. It is the more dangerous when armed, as it usually is, with sincere conviction. It is a spirit whose wrath must be turned away by the soft answers of a sweet reasonableness. It can be exorcised only by invoking the Genius which watched over our infancy and has guided our development— a good Genius— still potent let us believe — the American spirit of civil and religious liberty. Our institutions were not devised to bring about uniformity of opinion; if they had we might well abandon hope. It is important to remember, as has well been said, "the essential characteristic of true liberty is that under its shelter many different types of life and character and opinion and belief can develop unmolested and unobstructed."

On Dramatic Poetry (1758)

RFC (Request for Comments) document: RFC 791 http://www.ietf.org/rfc/rfc0791.txt, Internet Protocol (September 1981)
This is often mistakenly attributed to Jon Postel, but it is actually a very slight variation on a quotation from John Shoch; both RFC-791 and its earlier version RFC-760 include, at the point in the text where this passage appears, a reference to Shoch's paper Inter-Network Naming, Addressing, and Routing, which is the original source of this observation.
Misattributed

Source: Uglies

Source: 1980s–1990s, Knowledge and Decisions (1980; 1996), Ch. 1 : The Role of Knowledge

As quoted in "Atom Energy Hope is Spiked By Einstein / Efforts at Loosing Vast Force is Called Fruitless," Pittsburgh Post-Gazette (29 December 1934); it was only after the breakthroughs by Enrico Fermi and others in producing nuclear chain reactions that the use of nuclear power became plausible.
1930s

David Letterman http://thinkprogress.org/2008/08/01/mccain-anthrax-iraq/ (18 October 2001), linking anthrax attacks in the U.S. to Iraq.
2000s, 2001

IEN (Internet Experiment Note) document: IEN 19 http://postel.org/ien/txt/ien19.txt, A note on Inter-Network Naming, Addressing, and Routing (January, 1982)
This quotation is often erroneously attributed to Jon Postel.

Source: Dictionary of Burning Words of Brilliant Writers (1895), p. 619.