Black's Definition of Natural Law

This expression, "natural law," or jus naturale, was largely used in philosophical speculations of the Roman jurists of the Antonine age, and was intended to denote a system of rules and principles for the guidance of human conduct which, independently of enacted law or the systems peculiar to any one people, might be discovered by the rational intelligence of man, and would be found to grow out of and conform to his nature, meaning by that word his whole mental, moral, and physical constitution. The point of departure for this conception was the Stoic doctrine of a life ordered "according to nature," which in its turn rested upon the purely supposititious existence, in primitive times, of a "state of nature;" that is, a condition of society in which men universally were governed solely by a rational and consistent obedience to the needs, impulses, and promptings of their true nature, such nature being as yet undefaced by dishonesty, falsehood, or indulgence of the baser passions. In ethics, it consists in practical universal judgments which man himself elicits. These express necessary and obligatory rules of human conduct which have been established by the author of human nature as essential to the divine purposes in the universe and have been promulgated by God solely through human reason.

And the relevant part of Black's definiton of natural:

...and then it means proceeding from or determined by physical causes or conditions, as distinguished from positive enactments of law, or attributable to the nature of man rather than to the commands of law, or based upon moral rather than legal considerations or sactions.

Seems mushy, eh? That's just the thing. A commentor or Judge can rattle off that a common law doctrine derives from natural law principles without giving clear reasons why. Studying the evolution of the common law is much more accurate a practice than deciding a case on common law (read natural law)...because common law evolves. In that common law evolves, there must not be a clear source of natural law. Hence, one must recognize the inevitable postmodern (not-quite realist critique) that natural law is judge-made. If the source were clear (if a Judge could say X wins because God, the source of natural law and thus the definer or the common law at work here says so) then we would not have evolving common law. Unless our conceptions of God's will changes (if we see God as the source). More sensibly, human reason is the source...and that, we know, changes with the days.