Here is one example of how a regress of explanation can terminate. Suppose you and I are old friends who haven't seen each other in years, and we happen to run

Here is one example of how a regress of explanation can terminate. Suppose you and I are old friends who haven't seen each other in years, and we happen to run into each other while we are both on trips to Japan. The answer to the question, "Why did we meet in Japan?" is that it was a coincidence -- which is a kind of way of saying there is no interesting explanation of precisely this fact (though there will certainly be explanations of causes of this coincidence such as our respective trips to Japan). You might try to generate a sort of regress by asking, "Why did this coincidence happen?", but the proper way to answer this question is to say that there aren't really reasons why specific coincidences happen, as opposed to other possible coincidences that didn't happen, though by further study of the general nature of coincidences and of this coincidence in particular, we might come to a more exact appreciation of how "big" a coincidence it is. Now you might accept this example but say that the PSR doesn't apply to "everyday" facts like the one in the example but only at some more fundamental level of description. There is, perhaps, some precise configuration of fundamental particles that is the microphysical basis of the fact that we met in Japan, and is natural to explain the existence of that precise configuration of particles in terms of its causal antecedents -- the precise configurations of particles that existed earlier. If you simply accept determinism, as most Enlightenment philosophers did, you get a kind of PSR at this level. But of course there is a regress lurking here since you can try to follow the causes back to the beginning (and you can also get a problem about PSR by asking about the reason for the laws themselves that govern the causal relationships). At this "cosmological" level, most philosophers would probably accept something like brute facts, but there is undeniably something very offensive about this idea, and I guess a reason why Leibniz is interesting is that he really resisted it.