“A clever child brought up with a foolish one can itself become foolish. Man is so perfectable and corruptible he can become a fool through good sense.”

La passion fait souvent un fou du plus habile homme, et rend souvent les plus sots habiles.
Variant translation: Passion often makes a fool of the cleverest man and often makes the most foolish men clever.
Maxim 6.
Reflections; or Sentences and Moral Maxims (1665–1678)

The God-Seeker (1949), Ch. 3

Source: The Final Empire

Bacchæ l. 480
Variant translation: To the fool, he who speaks wisdom will sound foolish.
Variant translation: He were a fool, methinks, who would utter wisdom to a fool. (translated by Edward Philip Coleridge)
Variant translation: Wise words being brought to blinded eyes will seem as things of nought. ( translated by Gilbert Murray http://www.gutenberg.org/files/8418/8418-h/8418-h.htm)
Source: The Bacchae

"1901", p. 76. Sometimes misquoted as "If fifty million people say a foolish thing, it is still a foolish thing". Sometimes misattributed to Bertrand Russell or Anatole France
A Writer's Notebook (1946)

W. Somerset Maugham, A Writer's Notebook (1949), entry for 1901
Sometimes misquoted as "If fifty million people say a foolish thing, it is still a foolish thing."
Sometimes misattributed to Anatole France
Note that Russell does say something similar in Marriage and Morals (1929): "The fact that an opinion has been widely held is no evidence whatever that it is not utterly absurd; indeed in view of the silliness of the majority of mankind, a widespread belief is more likely to be foolish than sensible."
Misattributed

The Pale Horse (1961)

Un sot savant est sot plus qu'un sot ignorant.
Act IV, sc. iii
Les Femmes Savantes (1672)