Dana McCourt at The Edge of the American West has a short series of posts on Leibniz and Spinoza, based partly on The Courtier and the Heretic by Matthew Stewart. This is great stuff, the kind of thing blogs do better than anything -- bite-sized interesting pieces that stand by themselves, just because. (And the cheap chronological hook that November 18 was the day in 1676 when the two met in the Hague.)
Scientists think of Leibniz as Newton's rival in inventing calculus, and barely think of Spinoza at all. But they were both among the most influential philosophers of all time.
Leibniz published books and treatises, but much of what we know of his philosophy comes in the form of letters. I’ve joked that he invented the calculus on the back of a cocktail napkin in the corporate lounge while his flight from Paris to Hanover was delayed, and that of course was an exaggeration for comic effect. It wasn’t the calculus, but a dialogue on theology, and it was on a yacht from London to Rotterdam that was held fast in port by headwinds.
The two men came started from different launching points, but ended up arriving at very similar philosophies.
Spinoza’s naturalism lead him to atheism, but Leibniz came to Spinoza via his theism. That is, Leibniz found himself desperately trying to come up with an argument that showed that his own philosophy was not threatened by the spectre of Spinozism, but his philosophical commitments, especially those concerning the nature of God, meant his options were limited.
Foremost among those commitments was the Principle of Sufficient Reason: the idea that nothing is "just because," there is always an intelligible reason for everything feature of the world. It sounds innocent enough, but takes you to dangerous places if you buy into it with all your heart. As far as I can tell, the PSR is not especially popular in respectable philosophical circles these days, but it is still hanging in there. It's basically the foundation for Paul Davies's claim that any respectable laws of physics must have a good reason for being the way they are. I don't agree, myself; it might be true, but I'm very open to the possibility that the final product of our investigation into the ultimate workings of nature will be a set of rules that could easily have been different, but they simply are they way they are. At the very least, I would strongly defend the proposition that we should be open to this possibility; whether or not there is a small set of brute facts about the universe that lack any underlying justification, there is certainly no good reason to deny that scenario on the basis of pure thought, before we know what the ultimate rules actually are. At a more casual level, the PSR shows up in the common belief that everything happens for a reason. That's where the pernicious side of this purportedly sunny philosophy rears its head: if everything has a purpose, even the most terrible random events require an explanation, and from there it's a short road to the urge to put the blame on someone. Or, on the flip side, to kill 'em all and let God sort them out. One day, when human beings have universally adopted an enlightened materialist view of the cosmos and have developed a corresponding system of ethics and morality, an important piece of the puzzle will be an acceptance of randomness and contingency. All is not for the best, in the best of all possible worlds, and that leaves it up to us to try to make things better.