Inference to the best explanation - casually cited, a key tool for investigating the world. But Leibniz would ask us what we mean by "best". Do we mean sort of best? Best for what? Do we mean best-best? It's clear that only one thing can be the best: absolute perfection. Therefore inference to the best explanation always yields absolute perfection as the reason for what we observe. And, the promise goes, this is not a pseudo-explanation yielding no useful consequences, but the most fruitful explanation of all.
Like many of his predecessors (but few of his successors), Leibniz ethicizes reality itself: more reality means more goodness, so the best is the most real. Absolute perfection is therefore a concept that invalidates the distinction between goodness and what is factually the case. To contemporary eyes this joining of fact and value can look indefensible - not only for its intellectual awkwardness but for its ethically troubling implication that things could not be any better than they actually are.
Tonight we'll continue our increasingly glacial read-through of Leibniz's Monadology, starting with paragraph 36. We'll start once again with that notorious conversation-starter, the Principle of Sufficient Reason. Leibniz takes risks like few other thinkers. With the PSR, is he making a transparently false claim about human reasoning? A transparently unjustified inference from how we think to what must be the case? Possibly both. But what we must not do is assume that we know what he means by "God". Some lines of evidence suggest that he didn't know what he meant - beyond knowing that, whatever it is, "God" is the most fruitful concept of all.