I discovered a 2018 lecture handout of Brandom’s that offers a few more clues to his particular notion of modernity. There, Rousseau’s specific version of the social contract is singled out as achieving a synthesis of attitude-dependence of normative statuses (from natural-law voluntarism) with the status-dependence of normative attitudes (from what Brandom calls the perfectionist tradition of self-governance). Hegel is cited as saying that “the principle of freedom emerges in Rousseau”. I am not a Rousseau scholar, and lack familiarity with a specific “perfectionist” tradition. The Hegel quote is of the sort that used to give me no end of trouble, as seeming to imply an endorsement of voluntarism.
Brandom says that Rousseau’s notion of the sovereign applies the traditional subordination-obedience model of normativity with a unique twist — since Rousseau’s sovereign includes all the citizens, the commanding superior subject and the obedient subordinate subject coincide. I’ll hazard my own characterization of Rousseau as an antirationalist voluntarist antiphilosopher with a commendable if naive notion of the natural equality of persons and an insight into the moral difficulties associated with the notion of property that are still insufficient to make his overall position endorsable in the form in which he presented it.
Granting the huge impression Rousseau made on Kant, and Hegel’s endorsement of Kant’s response, I still want to say the important thing is what Kant and Hegel did to transform naive Rousseauian free will and equality into something viable, moving away from the subordination-obedience model altogether.
(The French Encyclopedists and the Spinozist movement evinced an actually more universal concern for social justice than Rousseau and his followers, while putting a positive value on reason and opposing voluntarism. This seems to me like a better antecedent than the one Kant and Hegel actually recognized. Going further back, Aristotle recognized inequality as a contingent fact but already thought something like mutual recognition applied between friends, while implying that members of a polity should treat one another like friends. Looking forward, I still think Plato and Aristotle have more in common with Kant and Hegel, and offer more that should be genealogically recollected, than someone like Rousseau; and that, based on the History of Philosophy lectures, Hegel would agree.)