There are unknown unknowns

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Science and technology

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What Wikipedia says

"There are unknown unknowns" is a phrase from a response United States Secretary of Defense Donald Rumsfeld gave to a question at a U.S. Department of Defense (DoD) news briefing on February 12, 2002, about the lack of evidence linking the government of Iraq with the supply of weapons of mass destruction to terrorist groups.

Excerpt from the Wikipedia article “There are unknown unknowns”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: R. D. Ward, Public domain, via Wikimedia Commons.