To my mind, the concept of (a?; the?) "natural law" is just too vague and amorphous to serve as the ethic of justice, i.e., the ethic for governing the governance of society. For one thing, the identification of "human nature" is too much a matter of personal predilection.

On the other hand, "observation" does point the way to a valid ethic of justice. It can be observed that human beings have no choice but to effect choices, i.e., choose from among perceived alternatives and take action to bring that choice to fruition. [Warren J. Samuels all but defined "social power" as the ability to effect choices.] Approaching justice from that observation makes the subject of justice the actions of people (which can include 'speech acts'), and nothing else.

That observation means that choosing integral to being human. That means that recognizing people as fellow humans requires respecting their capacity to choose--beginning with choosing whether/how/to what extent to be involved whenever any choice is being effected. Including the affects on others that actions taken in effecting choices can have, that boils down to a handful of absolute prohibitions: no killing, harming, coercing, stealing, or manipulating (lying, cheating, etc.) in effecting any choice.

So the ethic of justice is 'mutual respect in effecting choices'. Since the political process is the process of effecting choices for the community as a whole and a requirement of mutual respect follows from a belief in human equality, mutual respect in effecting choices is already the ethic that governs the democratic political process with which we are familiar (as a process--people's personal actions within that process should be directly governed by the ethic of justice). It can be applied to the economy as a societal process--with, again, the ethic applying to all actions of people as individuals within that sphere of human activity.

Liberalism, with its relation to a naturalistic approach to justice, did represent an advance in our understanding of justice. This ethic can take that understanding "Beyond Liberalism."