“People who know little are usually great talkers, while men who know much say little.”

"Why the AR-15 Is So Lethal", The Atlantic (7 November 2017)

In "Some Thoughts on Public Discourse" http://effective-altruism.com/ea/17o/some_thoughts_on_public_discourse/, February 2017

Les médecins administrent des médicaments dont ils savent très peu, à des malades dont ils savent moins, pour guérir des maladies dont ils ne savent rien.
This attribution to Voltaire appears in Strauss' Familiar Medical Quotations (1968), p. 394, and in publications as early as 1956 http://books.google.pt/books?id=lCtCAQAAIAAJ&q=%22Doctors+are+men+who+prescribe+medicine+of%22&dq=%22Doctors+are+men+who+prescribe+medicine+of%22&hl=pt-PT&sa=X&ei=mbnWUsvDIfTB7Aaw_YD4Dw&redir_esc=y; the quotation in French does not, however, appear to be original, and is probably a relatively modern invention, only quoted in recent (21st century) published works, which attribute it to "Voltaire" without citing any source.
Attributed

This attribution to Voltaire appears in Strauss' Familiar Medical Quotations (1968), p. 394, and in publications as early as 1956 http://books.google.pt/books?id=lCtCAQAAIAAJ&q=%22Doctors+are+men+who+prescribe+medicine+of%22&dq=%22Doctors+are+men+who+prescribe+medicine+of%22&hl=pt-PT&sa=X&ei=mbnWUsvDIfTB7Aaw_YD4Dw&redir_esc=y; the quotation in French does not, however, appear to be original, and is probably a relatively modern invention, only quoted in recent (21st century) published works, which attribute it to "Voltaire" without citing any source.
Original: (fr) Les médecins administrent des médicaments dont ils savent très peu, à des malades dont ils savent moins, pour guérir des maladies dont ils ne savent rien.

Source: The Souls of Black Folk (1903), Ch. XII: Of Alexander Crummell

Part V, Ch. 3
O Pioneers! (1913)

We the People interview (1996)
Context: The latent function of schooling, that is, the hidden curriculum, which forms individuals into needy people who know that they have now satisfied a little bit of their needs for education, is much more important... The idea that people are born with needs, that needs can be translated into rights, that these rights can be translated into entitlements, is a development of the modem world and it's reasonable, it's acceptable, it's obvious only for people who have had some of their educational needs awakened or created, then satisfied, and then learned that they have less than others. Schooling, which we engage in and which supposedly creates equal opportunities, has become the unique, never-before-attempted way of dividing the whole society into classes. Everybody knows at which level of his twelve or sixteen years of schooling he has dropped out, and in addition knows what price tag is attached to the higher schooling he has gotten. It's a history of degrading the majority of people.

On the Ergodic Behavior of Dynamical Systems (LA-2055, May 10, 1955) in [Stanisław Marcin Ulam, Analogies between Analogies, The Mathematical Reports of S.M. Ulam and His Los Alamos Collaborators, University of California Press, 1990, http://ark.cdlib.org/ark:/13030/ft9g50091s/]

Letter to Nikita Khrushchev after JFK assassination, as quoted in One Minute to Midnight: Kennedy, Khrushchev, and Castro on the Brink of Nuclear War (2009) by Michael Dobbs.