Rousseau is often read as engaged in the same project as Hobbes and Locke, namely, employing the state of nature myth as a way to distinguish what humans have by nature - and so what doesn't need political justification or explanation in political philosophy - and what is gained by governance - and so what does require political explanation or justification. Indeed, Rousseau in the Discourse seems to be claiming Hobbes and Locke just didn't take the state of nature myth seriously enough, and if they had, they'd have started with primitive man, a creature full of possibility - in the form of potential dispositions - but lacking instincts one might find among animals. Among these potential dispositions is the ability to reason, but importantly, this isn't something primitive man comes equipped with; it must be cultivated at the outset like any disposition. On this way of reading Rousseau, he incurs a significant explanatory burden, having to - like Hobbes and Locke before him - provide a plausible story for how one gets from the so-called primitive individuals to modern individuals. Hobbes and Locke claim inhabitants in the state of nature come equipped with the ability to reason, which makes the transition stories offered more plausible. Rousseau’s explanation, however, seems much less plausible…
Simply put, it is not at all clear how a non-reasoning entity, only perceiving tokens, lacking language, with its limited drives, ever develops such characteristics as we find them in modern individual. Of course, it might simply be argued this transition must have occurred, since modern individuals clearly exist. But while it's obvious modern individual exist (hello!), it's not so clear they made this transition from primitive individuals. These observations, along with what I take to be Rousseau's rather dismissive putative explanation of the transition, coupled with his emphasis on the problems one finds in moderns society while suggesting we can't return to our primitive state, suggest to me we should read Rousseau in a slightly different way. Namely, as providing a reductio argument against the very idea of employing a state of nature myth as Hobbes and Locke had. On this reading, Rousseau is not engaged in the same project as these philosophers. Rather, he's undermining a core starting position of their respective theorizing. He's then turning to concrete problems that come from governance to indicate since we can't separate ourselves from governance, we should focus on ourselves under governance.
More specifically, the argument I have in mind is the following:
Rousseau’s Reductio of the State of Nature Myth
(1) SUPPOSE: The state of nature myth reveals basic human nature
(2) If (1), then primitive individuals exhibit basic human nature
(3) If primitive individuals exhibit basic human nature, then transition from primitive individual to modern individuals is possible
(4) Hence, transition from primitive individuals to modern individuals is possible
(5) Transition from primitive individuals to modern individuals is not possible
(6) Hence, the state of nature myth does not reveal basic human nature
The first premise is assumed by Hobbes and Locke. (2) is Rousseau’s contribution, since he claims taking the state of nature myth seriously entails primitive individuals - as he understands them - will be the appropriate sort of entity to reveal basic human nature. If this is so, then the transition to modern individuals must be possible, i.e. (3), and so by logic the consequent follows, i.e. (4). But I claim Rousseau believes (5) as well, i.e. he doesn’t think it is possible for this transition to occur. If so, then since we’ve derived a contradiction from our assuming (1), we must take that back, and so we have (6) which is the negation of (1).
That said, premise (5) is – I think – ambiguous, and this may make it seem less plausible than it is. Consider the following three ways to read “possible” in this premise:
Metaphysical – As a matter of fact, independent of what humans think about it, it is impossible for primitive individuals to transition to modern individuals
Epistemic – The putative transition from primitive individuals to modern individuals is not something we could know about
Nomological – It is incompatible with the laws of nature that it’s possible for primitive individuals to transition to modern individuals
If we read (5) in the metaphysical sense, then it’s clearly false. The world doesn’t care what we know or don’t.
If we read (5) in the epistemic sense, then it seems (5) might be true, but then I think Rousseau would reject (3). Simply put, there’s no reason to think there is a connection between the existence of primitive individuals and what we could or couldn’t know about them.
However, if we read (5) in the nomological sense, then it seems (5) might be true, and (3) might be as well. With respect to the former, Rousseau would be claiming the laws of nature rule out this transition, and he might motivate this by reflecting on the difficulties inherent in bridging a non-reasoning creature to the sort of reasoning creatures we are, among other things. Additionally, (3) might still be accepted since there’s plausibly a link between primitive individuals revealing basic human nature and nomological possibility.
I’ll let you sort out whether you find the preceding convincing or not.