Even if we were to suppose that there were this terrible right to kill everyone, I maintain that neither a person enslaved during wartime nor a conquered people bears any obligation whatever toward its master, except to obey him for as long as it is forced to do so. In taking the equivalent of his life, the victor has done him no favor. Instead of killing him unprofitably, he kills him usefully. Hence, far from the victor having acquired any authority over him beyond force, the state of war subsists between them just as before. Their relationship itself is the effect of war, and the usage of the right to war does not suppose any peace treaty. They have made a contract. Fine. But this contract, far from destroying the state of war, presupposes its continuation.
Thus, from every point of view, the right of slavery is null, not simply because it is illegitimate, but because it is absurd and meaningless. These words, slavery and right, are contradictory. They are mutually exclusive. Whether it is the statement of one man to another man, or of one man to a people, the following sort of talk will always be equally nonsensical. “I make an agreement with you that is wholly at your expense and wholly to my advantage; and, for as long as it pleases me, I will observe it and so will you.”
– Jean-Jacques Rousseau, On the Social Contract (On Slavery, Chapter 4)