The meaning of natural law has changed over time. The general idea: what rights (what we must be given) and oughts (what we must do) pop straight out of nature?

For Hobbes, natural law is determined by natural rights, while the reverse held prior to him. The essential natural (human) right: “to use his own power, as he will himself, for the preservation of his own Nature; that is to say, of his own Life; and consequently, of doing any thing, which in his own judgement, and Reason, he shall conceive to be the aptest means thereunto.” (Leviathan. 1, XIV) This is determined by considering the human organism and its internal logic taken in isolation from any system of game rules. If we take the basis of rights to be the biological unit, then we take its good as the standard of allowance. Now, rights essentially includes the notion of others since rights concerns curtailment of power. Taking the individual alone removes this curtailment. What is left is the rationally self-interested monad whose telos is maximizing its continuance. If another is an obstacle or a threat, killing is right.

This, it seems to be, is an analytic truth and a proper philosophical starting point since it is akin to the greatest starting point in all of philosophy, the cogito. It is akin to the Anthropic Principle of cosmology—the necessary conditions for the possibility of asking this question must exist (as I am asking the question).

Click here for an excursion that compares three famous self-reinstating (properly metaphysical) principles.