Habermas bases legitimacy on discourse that includes both private autonomy and public autonomy. He does not believe that you can have legitimacy which ignores either of these.
Natural law is that law which seems to come from a higher source beyond the laws written by man.
Kant identified that source as morality. Habermas objects that morality is tied to specific religious and spiritual beliefs that in the age of phenomenology are subject to question. Habermas objects that the ethics approach is no longer feasible in the face of global pluralism. He suggests that the legitimacy we tend to see in natural law must come in the twenty-first century through democratic discourse and agreement on those values in which we can all believe. Habermas also insists that we must all participate in such democracy.