Why the Principle of Sufficient Reason is Self-Evident and its Criticisms are Self-Defeating

make the PSR the fourth law of logic again

Traditionally, the laws of thought have been limited to the law of identity, law of non-contradiction, and the law of the excluded middle. However, some have argued for adding a fourth member to this trio of foundational principles: the Principle of Sufficient Reason (PSR).

There are a few versions of the PSR, but for clarity, this article will defend the version of the PSR which states that all contingent facts have reasons for their existence. For every contingent fact ("X"), there is a sufficient condition ("Y").

A mathematical truth like (1+2=3) is a necessary truth rather than a contingent one, as for (1+2) to equal anything else would be impossible. Meanwhile, a truth like "Paris is the capital of France" is only a contingent truth, since whether Paris became the country's capital or another French city would have depended on prior events in reality. Marseille or Lyons could have become the capital if events played out differently.

While necessary truths exist in all possible worlds, our contingent truths exist only in this one—but everything still could have turned out differently, so we believe. Yet this assumption of possibility is one we impose on the world. If the PSR is true, then everything would be necessary. And because the PSR is self-evidently true, everything is necessarily true. People dislike this conclusion of the PSR, but if the PSR is true, we shouldn't avoid it by denying the self-evident.

Why the PSR is Self-Evident

How can you argue against the PSR? Well, you'd need arguments or reasons for rejecting the principle. Yet this approach accepts the PSR already. Any good argument requires that one provide reasons for others to accept it. And by giving reasons against the PSR, one is giving reasons for the PSR by accepting the PSR. Once we subject the PSR to reason, we need sufficient reasons to determine whether we need sufficient reasons! The PSR is self-evident and axiomatic.

For us to require that our truths be grounded on sufficient reasons is just to accept the PSR. Aristotle was able to prove that rejections of the law of non-contradiction (LNC) themselves required accepting the LNC in order to say anything definite. Similarly, rejections of the PSR on the basis of reasons are just as self-defeating. The PSR is as axiomatic as the LNC.

The PSR serves as the very basis of our judgments of reality. And because of the PSR, we can critique positions in philosophy that rest on unfounded assumptions.

But if you want some harder evidence, see here, here and here for findings on how fundamental the PSR is in all of our ordinary judgments. You may say you "reject" the PSR, but in reality, you aren't going to accept brute facts (contingent facts that are true without dependency) either. If you can't find your keys, you don't consider the "brute" possibility that your keys grew legs and walked away. Rather, you'd know your keys will be where you last left them (the sufficient reason for their place in the world).

Whenever we ask a question, we don't accept answers like "That is just how it is," or "It's just a brute fact." We demand explanations. In fact, the whole reason we ask questions is to discover these underlying explanations, which we already presume to exist.

Deniers of the PSRs resemble so-called "global skeptics," who may say that they doubt their knowledge on paper—however, if you were to examine their lives, you'd see they, too, operate under the assumption of having (or not having) knowledge—no different than anyone else. They will never say as much, which is why actions count more than words.

The same goes for the PSR. People may say that the universe is fundamentally random and physical events lack true explanation, but they will still navigate through life by asking "why?" questions and would never accept "just cuz its brute," as an answer. In fact, all of science presumes the PSR. The search for fundamental explanations operates on the assumption that those explanations exist. The PSR is how we see the world.

You might try to argue that even though reasons against the PSR are self-defeating, there still can be contingent truths that lack an explanation, independent of whether or not we accept the PSR. We cannot be forced to accept the PSR just from clever equivocation.

Yet, the PSR is not empirical, it is axiomatic. Whether or not we accept the PSR will determine how we will examine the world, not the world itself, and we cannot see the world outside our axioms of examination (the “laws of thought”). And to establish the possibility of ungrounded contingent truths (i.e., "brute facts") would first require rejecting the PSR. If we can't first reject the PSR, then, in principle, all contingent truths must have sufficient reasons.

But if, in principle, contingent facts require sufficient reasons, then no fact can be classified as truly brute. So although we don't know the specific sufficient reasons for a certain contingent truth, those sufficient reasons would still have to exist—we just wouldn't know them yet. The PSR lets us be intellectually humble by putting the burden of a missing structure on our own model rather than reality itself.

Criticisms

I feel justified to stop writing here. The PSR is truly undeniable. So much so that I view the PSR as equally self-evident as the law of identity (i.e., 1=1). But I can address the popular counter-arguments against the PSR.

Quantum Indeterminacy: One argument is that quantum indeterminacy in physics has questioned our assumption of an underlying logical structure to the universe, leading some to conclude that the physical is fundamentally random. But it's more parsimonious to just conclude that our model of the physical universe is incomplete, and axiomatic truths shouldn't suffer due to this incompleteness.

It would be surprising to find that millenniums of human advancement and understanding of the physical world would end in the conclusion that everything is just brute and random and that all of our supposed "explanations" were pure, fortunate, accidents.

We should reserve judgment on true quantum randomness and still posit the existence of a structure that we haven't fully grasped. Its self-evident that there is more science to do. Otherwise, science wouldn't be worth doing, as it's futile to search for underlying explanations of the world when those explanations aren't actually there.

Neccesititarianism: Peter Van Inwagen is credited for his critique of the PSR by flagging its implications for necessitarianism. His argument, oversimplified, is that if all contingent facts are necessarily created out of sufficient reasons, then all contingent facts would have to be "necessary facts." Therefore, there cannot be any contingent facts, only necessary ones.

I've had no problem with this application and am surprised by the weight its received, leaving some to drop the PSR entirely. But this supposed critique ignores that philosophers have already accepted some sense of determinism, per the 2020 Phil Survey showing that "soft determinism" (i.e., compatibilism) is the most dominant view in the free will debate.

“Soft determinists" posit that a series of necessary events nonetheless allows for free will—just as the PSR still allows for contingent truths. So like how philosophers can reconcile free will with determinism, contingent truths can similarly be made compatible with necessitarianism.

The supposed "necessitarian" critique is also a feature, rather than a bug, of the PSR. Necessitarianism is consistent with science's presumption of fundamental explanations for the universe, or laws of nature (otherwise, what would be the purpose of searching for explanations if they aren't there)? Whenever we examine and study the world for answers to our "why" questions, we accept the PSR.

Infinite Regress: Another criticism of the PSR is that its demand for sufficient reasons is infinite. The PSR may lead us to require a never-ending chain of contingent facts, and because infinity can't exist in the physical world, the PSR can't be true. No one truly believes that there are infinite turtles; why then believe in infinite reasons?

But this infinite chain argument ignores necessary truths. Necessary truths, such as the truths of mathematics and logic, are self-evident and axiomatic. They don't need a reason to exist, as they are "givens" in every possible world. Physics is dedicated to mapping contingent physical events onto necessary truths through mathematics. Once you create the necessary model of reality, everything else becomes derivative.

Any other criticism: Even if you don't subscribe to any defense of the PSR I provided above, arguments against the PSR are still self-defeating. Using reasons not to accept the PSR is to accept the PSR. I'll summarize with the following:

1. To criticize the PSR is to give a reason against the PSR.

2. To give a reason against the PSR is to ground truth is reason.

3. To ground truth in reason is to accept the PSR.

4. So any criticism of the PSR must accept the PSR.

5. Therefore, all criticism of the PSR must be self-defeating.

The reasoning might be annoying, but it works. Try disproving reason without a reason.

The PSR is a powerful tool for any philosopher. The fact that only 35% of philosophers in the PhilSurvey accept the PSR is unfortunate. The PSR allows one to question ungrounded claims and seek underlying truth. And as I've argued here and here, the PSR even lets us disprove an omnipotent God.

Conclusion

We can believe in the PSR until we have a reason not to believe it, in which case we will be believing the PSR. It seems safe to just make the PSR a self-evident fourth law of logic, until given reasons otherwise.