In common speech, the word "theory" is often used to refer to ideas that have not been proven, but to scientists and philosophers it has a completely different meaning.

FORBES: Sciences and Medicine

In his speech, Mr Denham said disadvantaged white boys had more in common with their poor black classmates than with middle-class white people.

BBC: Minister says councils 'blind' to needs of white people

In a ground-breaking speech in parliament earlier this month, Mr Erdogan provoked tears when he spoke of the common pain of Turkish and Kurdish mothers who had lost sons in the conflict.

ECONOMIST: Turkey and the Kurds

In his keynote speech, Mr Miliband talked about the common bonds of the four countries that make up the United Kingdom.

BBC: Scotland politics

As strange as the case is, experts said it touches on a common challenge in law enforcement: deciphering intent without running afoul of the First Amendment right to free speech.

WSJ: Fact and fantasy collide in NYC cannibalism trial