The PSR is the Principle of Sufficient Reason, the most controversial premise of the LCA which we have defended here previously. We will here provide another argument for the PSR. That is, for the claim that for every contingent thing (or perhaps indeed every single thing, contingent or not) has a sufficient explanation.

The argument I intend to motivate is this: we should extend reasoning principles as far as possible, until we have a good reason. The PSR is a principle we use in many areas, and so we should extend it to all objects unless we can come up with a good reason why we should not.

Now we all know how we apply the PSR in physical situations. There is currently light hitting my eye, as an explanation I posit a computer monitor in front of me as an explanation. An apple falls from a tree, and we posit gravity. Celestial objects move across the sky, and we posit heliocentrism. In none of these cases do we say “perhaps there’s no explanation”, we always immediately begin looking for one. If there’s some phenomenon we can’t find an explanation for, we conclude that we aren’t smart enough, or our equipment isn’t precise enough. We assume that there is one we just haven’t found.

So now we might be tempted to say “The PSR only applies to physical phenomena”, and then we wouldn’t be forced to admit that God exists. And there may be some cause for this since intuitively, physical objects are the kinds of things which seem like they’re causally closed. But maybe other kinds of phenomena (if they exist at all) are “spooky” and not causally closed, so maybe there’s no PSR there.

Let’s consider another domain then: ethics. Take the famous trolly problem: suppose it turned out to be the case that it was morally wrong to pull the lever to save five while condemning one. Suppose this was morally impermissible. And an interested interlocutor would ask “Why?” Suppose we answered, “It just is, there’s no reason”. Is that an acceptable answer? Clearly not. And in fact many ethicists have spent their lifetimes looking for answers to such ethical question: what is right and what is wrong, and *why *are those things right and wrong. Again, there is the presumption of explanation.

Now a third domain: mathematics. We observe some mathematical phenomenon, and we ask “why”. That “why” often comes in the form of a proof, or at least a sketch of a proof. Often the “why” is actually *hidden* in the proof, almost as if the author of the proof went to great effort to obscure the deep intuition. But sometimes it’s on the surface. And this doesn’t only apply to theorems, we can ask why a theorem is true, but we can also ask why something more general is the case. Here’s one such question: why does the sum of the series of inverse square numbers involve a pi term? Pi has to do with circles, but this doesn’t appear to be related to circles. And indeed, there is an explanation. This kind of question is often asked by mathematicians, why is something the case? And often the answer is intuitively satisfying, deep, and can lead to new ways of thinking about problems. Much of mathematics, rather than hunting for proofs of theorems, is hunting for these more intuitive, conceptual explanations of a more general class of phenomena.

Now we go back to our PSR. It holds in the domains of physics, ethics, and mathematics. Our natural, everyday reasoning assumes that the PSR holds when we consider physical objects, abstract objects (such as numbers), and moral facts. So how might we restrict it? If we say “All physical, ethical, and mathematical phenomena have sufficient explanations”, but that seems extremely arbitrary. And again, we normally want to expand principles until we have a good reason not to. What good reason do we have for restricting the PSR to these classes of phenomena, but not others?

And we can take this argument even further. We can follow Della Rocca, and point out that unless you can give a sufficient reason for that restriction, you’ve begged the question. I covered that particular argument in the original post on the PSR, but it is worth reiterating because I think it is indeed a strong one.