The Spirit of Laws, Book II - Of Laws Naturally Derived From The Nature Of Government

There are three natural arrangements for government.

A republic rests supreme power in the people at large-- either in an aristocracy, or with everyone in a democracy.  The most important task in a republic is to have a fixed concept of suffrage-- carelessness in this respect led to the downfall of Rome.  Different societies would have different natural ways of determining the best method.  But even in a full democracy, it becomes necessary for an elected body to direct the passions of the people-- likely an aristocracy.

It is possible for a member of the aristocracy to gain a sudden wealth of great power-- necessary, even, in some cases.  Fine, as long as such periods under a magistrate are kept short and well-defined.  Better, though, for an aristocracy to lean in the direction of democracy than in the direction of monarchy.

Under a monarchy, power is vested in an individual, who then rules according to a body of law-- either pre-existing, or proposed by a legislative group (possibly, even if informally, the nobility).  Whatever the structure of this body, it serves as a check on the capriciousness of the monarch.

In despotic government, there is no body (of persons or of laws) for the supreme ruler to be checked by.  In most cases, the despot will appoint a vizier to handle mundane cases-- which only makes things worse.  The isolation from the public at large makes a despot even worse of a ruler-- and he doesn't even know it.

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I'm getting a very positive vibe out of Montesquieu-- he thinks mankind is doing pretty well, and generally issues have a way of working themselves out for the best.  This is a great contrast to what I read in a lot of contemporary debate, where all that gets discussed are the fundamental problems with societal arrangements.

I'm also thinking Montesquieu didn't do a lot of research-- around the world, or extensively historical-- in coming up with his list of possible systems.  Maybe every system that's ever been tried would fall into his categories, but it seems more likely that he's just describing the variations in his corner of the world.