“You can recognize a really good idea by the fact that its implementation seems impossible in the first place.”
Google shows that the internet often attributes this statement to Einstein, but never with a source. It does not occur in any book in Google Books.
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Albert Einstein 702
German-born physicist and founder of the theory of relativi… 1879–1955Related quotes

“An idea is nothing, its implementation everything.”
Attributed to Kronrod in: Landis, E. M., Yaglom, I. M., Remembering AS Kronrod, The Mathematical Intelligencer 24.1 (2002) p. 22-30
C. West Churchman (1979, p. 21) as cited in: Interfaces (1982) Vol 12, p. 12
1980s and later

“In fact, all kinds of men, and not merely architects, can recognize a good piece of work…”
Source: De architectura (The Ten Books On Architecture) (~ 15BC), Book VI, Chapter VIII, Sec. 10

Philosophical Fragments, P. Firchow, trans. (1991) § 116


Source: "Idea and Man in the Ideological Movement" (1954)