“Where all think alike, no one thinks very much.”
The Stakes of Diplomacy http://books.google.com/books?id=cyFMAAAAMAAJ&q=%22Where+all+think+alike+no+one+thinks+very+much%22&pg=PA51#v=onepage (1915)
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Walter Lippmann 14
American journalist 1889–1974Related quotes

“When everyone thinks alike not much gets thought.”
Ett annat Sverige är möjligt (2006), p. 18
Context: Now it happens to have been the Social Democrats who have regimented the important institutions in society, but it would have been dangerous whichever party it had been. Development and diversity depend on independent initiatives and competition. When everyone thinks alike not much gets thought.

“I think a lot of snowflakes are alike… and I think a lot of people are alike too.”
Source: American Psycho

After being asked "Does someone as sophisticated as you are submit yourself to the conspiracy theory of history?""
1980s, At The David Susskind Show (1980)

“We need not think alike to love alike.”
This attribution seems to have begun in the 1960s, and has been debunked at "Who really said that?" by Peter Hughes at UU World (15 August 2012) http://www.uuworld.org/ideas/articles/229844.shtml; previously misattributed in A Chosen Faith (1991) by John A. Buehrens; also in Unitarian Universalist Origins: Our Historic Faith by Mark W. Harris https://web.archive.org/web/20060101061859/www.uua.org/info/origins.html
Misattributed
Earth Made of Glass (1998)

Sermon 39 Catholic Spirit http://new.gbgm-umc.org/umhistory/wesley/sermons/39/ from the 1872 edition of Wesley's Complete Works - Thomas Jackson, editor
Sermons on Several Occasions (1771)

Meet the Press, September 25, 2005
"The next … months" in Iraq