“Men are foolish, are they not, Mademoiselle? To eat, to drink, to breathe the good air, it is a very pleasant thing, Mademoiselle. One is foolish to leave all that simply because one has no money — or because the heart aches. L´amour, it causes many fatalities, does it not?”

The Mystery of the Blue Train (1928)

Politics and the English Language (1946)

Source: The Book of the City of Ladies

"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

Address at the Congress of the Association of the Universities of the British Commonwealth, Montreal, September 1, 1958
Speaking Of Canada - (1959)

Source: Self-Consciousness : Memoirs (1989), Ch. 6; Gratia Dei sum quod sum translates to ”Thanks be to God that I am what I am”

Hercule Poirot
The Mystery of the Blue Train (1928)