[T]he law of nature stands as an eternal rule to all men, legislators as well as others. The rules that they make for other men’s actions, must, as well as their own and other men’s actions, be conformable to the law of nature, i.e. to the will of God, of which that is a declaration, and the fundamental law of nature being the preservation of mankind, no human sanction can be good, or valid against it.
Source: John Locke, Second Treatise on Government, Ch. 11