“One of the most popular superstitions consists in the belief that every man is endowed with definite qualities—­that some men are kind, some wicked; some wise, some foolish; some energetic, some apathetic, etc. This is not true. We may say of a man that he is oftener kind than wicked; oftener wise than foolish; oftener energetic than apathetic, and vice versa. But it would not be true to say of one man that he is always kind or wise, and of another that he is always wicked or foolish. And yet we thus divide people. This is erroneous. Men are like rivers—­the water in all of them, and at every point, is the same, but every one of them is now narrow, now swift, now wide, now calm, now clear, now cold, now muddy, now warm. So it is with men. Every man bears within him the germs of all human qualities, sometimes manifesting one quality, sometimes another; and often does not resemble himself at all, manifesting no change. With some people these changes are particularly sharp. And to this class Nekhludoff belonged.”

"How Not To Better Social Conditions" in Review of Reviews (January 1897), p. 39
1890s

"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

Antisthenes, 4.
The Lives and Opinions of Eminent Philosophers (c. 200 A.D.), Book 6: The Cynics

Nus ne puet estre trop parliers
Qui sovent tel chose ne die
Qui torné li est affolie,
Car li sages dit et retrait:
Qui trop parole, il se mesfait.
Source: Perceval or Le Conte du Graal, Line 1650.

Sisyphus, as translated by R. G. Bury, and revised by J. Garrett http://www.wku.edu/~jan.garrett/302/critias.htm
Variant translation: He was a wise man who originated the idea of God.

Written on the Bedchamber Door of Charles II, as quoted in The Book of Days : A Miscellany of Popular Antiquities (1832) by Robert Chambers, Viol. II, July 26, p. 126.