However, as it is impossible for the whole race of mankind to be united in one great society, they must necessarily divide into many; and form separate states, commonwealths, and nations; entirely independent of each other, and yet liable to a mutual intercourse. Hence arises a third kind of law to regulate this mutual intercourse, called "the law of nations;" which, as none of these states will acknowledge a superiority in the other, cannot be dictated by either; but depends entirely upon the rules of natural law, or upon mutual compacts, treaties, leagues, and agreements between these several communities: in the construction also of which compacts we have no other rule to resort to, but the law of nature; being the only one to which both communities are equally subject: and therefore the civil law very justly observes, that quod naturalis ratio inter omnes homines constituit, vocatur jus gentium.

Commentaries on the Laws of England, Book the First, William Blackstone, 1758