“You must know, then, that there are two methods of fighting, the one by law, the other by force: the first method is that of men, the second of beasts; but as the first method is often insufficient, one must have recourse to the second.”

Source: The Prince (1513), Ch. 18
Variant translations of portions of this passage:
Every one admits how praiseworthy it is in a prince to keep faith, and to live with integrity and not with craft. Nevertheless our experience has been that those princes who have done great things have held good faith of little account, and have known how to circumvent the intellect of men by craft, and in the end have overcome those who have relied on their word.
Ch. 18. Concerning the Way in which Princes should keep Faith (as translated by W. K. Marriott)
A prince being thus obliged to know well how to act as a beast must imitate the fox and the lion, for the lion cannot protect himself from traps, and the fox cannot defend himself from wolves. One must therefore be a fox to recognize traps, and a lion to frighten wolves.
You must know there are two ways of contesting, the one by the law, the other by force; the first method is proper to men, the second to beasts; but because the first is frequently not sufficient, it is necessary to have recourse to the second.
Context: How laudable it is for a prince to keep good faith and live with integrity, and not with astuteness, every one knows. Still the experience of our times shows those princes to have done great things who have had little regard for good faith, and have been able by astuteness to confuse men's brains, and who have ultimately overcome those who have made loyalty their foundation. You must know, then, that there are two methods of fighting, the one by law, the other by force: the first method is that of men, the second of beasts; but as the first method is often insufficient, one must have recourse to the second. It is therefore necessary to know well how to use both the beast and the man. This was covertly taught to princes by ancient writers, who relate how Achilles and many others of those princes were given to Chiron the centaur to be brought up, who kept them under his discipline; this system of having for teacher one who was half beast and half man is meant to indicate that a prince must know how to use both natures, and that the one without the other is not durable. A prince being thus obliged to know well how to act as a beast must imitate the fox and the lion, for the lion cannot protect himself from snares, and the fox cannot defend himself from wolves. One must therefore be a fox to recognise snares, and a lion to frighten wolves. Those that wish to be only lions do not understand this. Therefore, a prudent ruler ought not to keep faith when by so doing it would be against his interest, and when the reasons which made him bind himself no longer exist. If men were all good, this precept would not be a good one; but as they are bad, and would not observe their faith with you, so you are not bound to keep faith with them.... those that have been best able to imitate the fox have succeeded best. But it is necessary to be able to disguise this character well, and to be a great feigner and dissembler.