Famously in Spirit of the Laws (1748), Montesquieu identifies four main principles of the different kinds of government: virtue for a democratic republic, moderation for an aristocratic government, honour for monarchical government, fear for despotism. Montesquieu argues that a political system can only survive if it remains in harmony with appropriate principles. Montesquieu also deals with various overlaps, and individual particularities, so his approach is not as schematic as a bare summary might suggest, but that fourfold is one of the guiding thoughts in his work.
Rousseau’ Social Contract (1762) is strong influenced by Montesquieu. In passing, I would like to point out that Rousseau’s antagonist in the book is often Hugo Grotius, something that really should be more frequently discussed. Returning to Montesquieu, Rousseau has similar views to Montesquieu about the places of geography in discussing customs, laws and institutions; and they both look for broad archetypes of government, both looking to ancient republics for models, though never just a return to those republics.
It is in Book III, Chapter IX that Rousseau expands on the schema of principles offered by Montesquieu. Rousseau has various disagreements with Montesquieu’s approach along with the evident continuities. One of those is that virtue is a characteristic of all good government, not just democracies, Rousseau’s schema of types of government is 6 fold and rather than give distinct principles to each, he mentions the objects of peoples, war for the Spartans, letters for the Athenians, trade for Carthaginians and so on. These mores underly laws when properly formulated by a legislator. The different objects of government are compatible with general shared principles of law for Rousseau, but possible contradiction in his approach has emerged: particularism is elevated in laws, as is universalism.
This section of Rousseau reminded me of Nietzsche’s Thus Spoke Zarathustra (1883-85): Book I ‘A Thousand and One Goals’; Book III ‘New and Old Law Tables’. In these passages Nietzsche deals with a rather Rousseauesque list of the goals of different peoples in antiquity. He see them as emerging from a herd mentality which precedes individuation, and which then becomes established in law. The development of the ego towards a more individualising understanding threatens these goals from one side, as does the idea of humanity from the other side. The result is that humanity has no gaol, because different peoples have different goals. Humanity has no goal and it is difficult to say what humanity is. The second passage seems, in part, to be a reaction to that. Overcoming humanity is maybe the unifying goal. The goals of peoples or of individuals are inadequate for resisting what Nietzsche finds most negative in humanity: herd instinct, pity and hatred, ressentiment, etc. Though Nietzsche certainly sees value in some of the goals of peoples. In the second passage, the answer is in the particularly exultant (but possible self-parodyng) calls to overcome disgust, dance over heavy things, become affirmative, welcome contradiction and flux.
What Nietzsche says is not the same as Rousseau, it looks like a response which takes Rousseau very seriously. The idea of humanity as what lacks any fixity is in Rousseau. The struggle between different versions of the self in Rousseau (love of self, amour propre/self-love; master and slave etc) strongly anticipate Nietzsche, as does various ways he investigates the sources of negative attitudes in the passions, and the value of the individual perspective. Some of those themes go back to Montesquieu, though for some of them Persian Letters is the source rather than Spirit of the Laws.
Again, it’s not that these thinkers are saying the same thing, but that they respond to their predecessors in deep ways, and Rousseau is very much a predecessor for Nietzsche (again something than could be discussed more).