A man is his own easiest dupe, for what he wishes to be true he generally believes to be true.
— Demosthenes

A hypocrite despises those whom he deceives, but has no respect for himself. He would make a dupe of himself too, if he could.
William Hazlitt dupe quote

We assemble parliaments and councils, to have the benefit of their collected wisdom; but we necessarily have, at the same time, the inconvenience of their collected passions, prejudices, and private interests. By the help of these, artful men overpower their wisdom, and dupe its possessors; and if we may judge by the acts, arrets, and edicts, all the world over, for regulating commerce, an assembly of great men is the greatest fool upon earth.
— Benjamin Franklin

Education must have two foundations --morality as a support for virtue, prudence as a defense for self against the vices of others. By letting the balance incline to the side of morality, you only make dupes or martyrs; by letting it incline to the other, you make calculating egoists.
— Sebastian Roch Nicolas Chamfort

The philosopher is like a man fasting in the midst of universal intoxication. He alone perceives the illusion of which all creatures are the willing playthings; he is less duped than his neighbor by his own nature. He judges more sanely, he sees things as they are. It is in this that his liberty consists -- in the ability to see clearly and soberly, in the power of mental record.
— dupe quotation by Henri Frederic Amiel