Why should we obey the natural law? Some people say, "Because it is the law of our nature. To aspire to the good is not to conform ourselves to something alien to us, but to fulfill the requirements of our own flourishing."
Other people say, "Because it has divine authority. Our nature contains the possibilities of good that it does contain just because it is the creation of God."
But the mainstream of the classical natural law tradition has held that these two views are not competitors, but complements, because man is ordained to a twofold final good.
The goods available to us by the exercise of our natural powers, such as friendship, family, and knowledge, are good in themselves, yet not enough. Don't we know this by experience? One would think that the better these things are, the more satisfied we would be. Actually, the better they are, the more they stir us up to long for Something More, of which they are only glimpses.
Dante knew that. It was the whole point about Beatrice. The glory of this redeemed woman was reflected glory. We cannot finally be satisfied by any love short of divine love, by any vision short of the vision of God Himself.
Here is the catch. The supreme good of knowing God is available to us only if we are lifted beyond our natural powers by grace. Our nature is fashioned with the potentiality to receive this gift, but it cannot provide it for itself. All merely human attempts to partake of the divine nature, to become superhuman or transhuman, are merely idolatries which end in disaster.
Consequently, any account of human nature that treats God as just another natural good misses the point, and any natural law theory that stops with nature, rather than beginning with nature, is going to overlook the most important things about us.