Excerpted from The Prince, by Niccolo Machiavelli: XVIII

…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. One recent example I cannot pass over in silence. Alexander VI did nothing else but deceive men, nor ever thought of doing otherwise, and he always found victims; for there never was a man who had greater power in asserting, or who with greater oaths would affirm a thing, yet would observe it less; nevertheless his deceits always succeeded according to his wishes, because he well understood this side of mankind.

Therefore it is unnecessary for a prince to have all the good qualities I have enumerated, but it is very necessary to appear to have them.
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For this reason a prince ought to take care that he never lets anything slip from his lips that is not replete (filled) with the above-named five qualities, that he may appear to him who sees and hears him altogether merciful, faithful, humane, upright, and religious. There is nothing more necessary to appear to have than this last quality, inasmuch as men judge generally more by the eye than by the hand, because it belongs to everybody to see you, to few to come in touch with you. Every one sees what you appear to be, few really know what you are, and those few dare not oppose themselves to the opinion of the many, who have the majesty of the state to defend them; and in the actions of all men, and especially of princes, which it is not prudent to challenge, one judges by the result.
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One prince of the present time, whom it is not well to name, never preaches anything else but peace and good faith, and to both he is most hostile, and either, if he had kept it, would have deprived him of reputation and kingdom many a time.