In the nineteenth century, Karl Marx would look back critically on the group of thinkers that he referred to as "Robinsonards"—followers of Robinson Crusoe, the famous castaway from Daniel Defoe's novel who, with his own diligence, gradually builds up a completely self-sufficient life for himself in the solitude of a desert island. The problem with these thinkers, among whom Karl Marx also placed Adam Smith, was that, in their political and economic analysis, and speculative histories of humanity, they always began with the individual in isolation. Marx argued that, from the beginning of time, human history has always been collective—it's only with the dawn of capitalism, and the political philosophies it engenders, that the category of "the individual," as someone with natural rights (e.g. the right to property) comes into existence as a political concept. Rousseau, Smith and others simply project this contemporary political invention back into a fictional past to explain how human institutions were developed.
Just the same, the Discourse on Inequality is a crucial text for Marxism. Rousseau, like Marx, argues that human history is, in fact, the history of inequality, beginning with the division of labor. Thus, it is impossible to consider merely through rational argumentation which system of government is the most just, or the most effective—a common debate in Rousseau's time, as the first democratic revolutions against monarchy were taking place, and England already had a parliamentary system whose ostensible purpose was to check the king. While other thinkers tried to rationally compare the various systems, Rousseau was arguing that all systems of government have their roots in inequality. No existing social form, he said, is truly legitimate, because each simply puts the stamp of political legitimacy on what is a fundamentally illegitimate inequality between people. Inequality is not an effect of certain fixable problems in these systems, and it doesn't just influence certain social relations (the rich live in nicer houses than the poor) or imbalances of power (like between employer and employee). Rather, inequality determines all aspects of human life in society. This argument of Rousseau's decisively influenced Marx's analysis of capitalism. There was no such thing as just or egalitarian capitalism, Marx argued, because at its very core it was based on inequality, and worked to preserve and sharpen that inequality.
Also like Rousseau, Marx believed that the way out of this inequality was not back, to some idyll, but forward. Marx argued against utopians like Charles Fourier, who thought we had to create a new social arrangement completely different from the one that exists, effectively starting over. Similarly, Rousseau had distinguished himself from movements like the Physiocrats, who believed that we had to go back to the social forms of a rudimentary agricultural society. In his work The Social Contract—published 8 years after the Second Discourse—Rousseau imagined a society in which the individual would be identical with the general will. In this society, there would be a small government that would simply execute laws rather than pass them. This image of a society without inequality—without distinction between ruler and ruled—was echoed in Marx's dream of the day when workers would own the means of production. In this ideal arrangement, the state—the body that passes laws, rules over people, and punishes them—would "wither" away, uniting the will of the individual with that of everyone else.
