“Men are so simple, and so subject to present necessities, that he who seeks to deceive will always find someone who will allow himself to be deceived.”
Source: The Prince (1513), Ch. 18; translated by W. K. Marriot
Original
[…] sono tanto semplici gli uomini, e tanto ubbidiscono alle necessità presenti, che colui che inganna, troverà sempre chi si lascerà ingannare.
cap. XVIII
Il principe
Variant: Sono tanto semplici gli uomini, e tanto ubbidiscono alle necessità presenti, che colui che inganna, troverà sempre chi si lascerà ingannare.
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Niccolo Machiavelli 130
Italian politician, Writer and Author 1469–1527Related quotes

Variant: Men are so simple and so much inclined to obey immediate needs that a deceiver will never lack victims for his deceptions.

“It's not right to hurt or deceive someone who's already been hurt and deceived.”
Source: The Cider House Rules

“Men who can succeed in deceiving no one else will succeed at last in deceiving themselves.”
Miss Mackenzie, Ch. 13. (1865) · Project Gutenburg e-text http://www.gutenberg.org/etext/24000

Cosi all' egro fanciul porgiamo aspersi
Di soave licor gli orli del vaso;
Succhi ainari, ingannato, in tanto ei bene,
E da l'inganno iuo, vita ricere.
Canto I, stanza 3 (tr. Edward Fairfax)
Anthony Esolen's translation:
As we brush with honey the brim of a cup, to fool
a feverish child to take his medicine:
he drinks the bitter juice and cannot tell—
but it is a mistake that makes him well.
Compare:
Sed vel uti pueris absinthia taetra medentes / cum dare conantur, prius oras pocula circum / contingunt mellis dulci flavoque liquore, / ut puerorum aetas inprovida ludificetur / labrorum tenus, interea perpotet amarum / absinthi laticem deceptaque non capiatur, / sed potius tali facto recreata valescat.
When a doctor is trying to give unpleasant medicine to a child, he smears the rim of the cup with honey. And the child, not suspecting any trick, tastes it; and at first he is misled by the sweetness on his lips into swallowing it, however sour it is. But even though he is deceived, he is not distraught; and soon enough he gets better and regains his strength.
Lucretius, De Rerum Natura, Book I, lines 936–942 (tr. G. B. Cobbold)
Gerusalemme Liberata (1581)

Robert Aldrich, "Who's who in Gay and Lesbian History from Antiquity to World War II", London and New York, Routledge, 2001, p. 377.

Napoleon : In His Own Words (1916)
Context: It is a mistake, too, to say that the face is the mirror of the soul. The truth is, men are very hard to know, and yet, not to be deceived, we must judge them by their present actions, but for the present only.

“What was the point of spending your life with someone you were always looking for ways to deceive?”
How to Save Your Own Life (1977)