On the Necessity of the PSR
A Defense of the Principle of Sufficient Reason
Part I: Scope and Extent of Article
The Principle of Sufficient Reason (henceforth PSR) is a famous axiom of philosophy. It states that every event must have a sufficient condition for its reality—that is to say, every event must have an explanation. This excludes the possibility of something, say, coming from nothing. Something cannot just “pop” into existence without a cause. Indeed, the Scholastic dictum that “from nothing, nothing comes” is an expression of this very reality. The PSR was first verbalized by the rationalist polymath Gottfried Leibniz but implicitly undergirded Western philosophy up to that point. Ancient, Medieval, and Renaissance thinkers took it as a certainty.
This all changed with the advent of modern thinkers like Hume. Some sought to reduce the scope of the principle; others sought to eliminate it from philosophy altogether, viewing it as a relic of a bygone age. I would like to focus on one of the more forceful critics today—namely, Peter van Inwagen’s argument from incoherence.
Part II: Exposition of the PSR
The Principle of Sufficient Reason (PSR) states that for every fact or event P, there must exist a sufficient reason Q why P obtains rather than not. In other words, nothing happens without a reason or explanation—whether that explanation is found in a preceding cause (temporally) or in a grounding basis (ontologically). Put simply: every event or state of affairs must have an adequate explanation, either in something else or in itself.
There are stronger and weaker formulations of the PSR. The strong form claims that every contingent truth must have an explanation. Some restrict it only to certain domains (e.g., only to physical events or to causal chains within the universe). Others, like Leibniz, apply it to all truths, including the existence of the universe itself:
“Nothing happens without a sufficient reason; that is, nothing occurs without it being possible for someone who knows enough to give a reason why it is so and not otherwise.” (Monadology, §32)
According to modern analytic philosophy, a sufficient explanation expands beyond just a cause—when “cause” is defined in a restrictive, temporal sense. That is to say, all causes are explanations, but not all explanations are causes. One kind of explanation that isn’t a cause is a grounding relation—when one reality grounds another, or when some event depends ontologically on a prior basis. For instance, modern Platonists hold that every instantiation of a nature is dependent on the exemplar of that nature. Every instance of a triangle, for example, would be dependent on the universal nature of triangularity.
With this out of the way, let’s explore why someone might be motivated to reject the PSR.
Part III: Why Deny the PSR
To many, including myself, the PSR is an intuitive principle necessary for daily functioning. Never do we, as ordinary people, worry about something popping into existence out of nothing. It simply isn’t something we encounter. So what could motivate someone to deny something so evident to the senses? I want to examine one of the strongest objections: Peter van Inwagen’s argument from incoherence.
One of the most influential critiques of the PSR comes from philosopher Peter van Inwagen, who argues that it leads to contradiction when applied to the totality of contingent facts. His argument appears in The Place of Chance in a World Sustained by God (2010).
He argues that the set of all contingent truths must be brute—or have no explanation—on pain of contradiction. His argument runs as follows:
Suppose the PSR is true.
Then the BCCF (Big Conjunctive Contingent Fact) must have an explanation (by PSR).
The explanation of the BCCF must be a true proposition.
Every true proposition is either necessary or contingent.
The explanation of the BCCF cannot be necessary, because a necessary proposition cannot explain a contingent one.
Therefore, the explanation must be a contingent truth.
But since the BCCF is the totality of all contingent truths, its explanation—being contingent—must itself be one of the conjuncts of the BCCF.
Therefore, the BCCF contains a truth that explains the BCCF.
Thus, a contingent proposition explains itself (since it is part of the very thing it explains).
But no contingent proposition can explain itself.
Therefore, the PSR must be false by contradiction.
Let’s unpack this. According to van Inwagen, something is necessary if it cannot fail to exist—if it is always true. Contingent things, by contrast, can fail to exist; they do not exist by their very nature.
Van Inwagen assumes the truth of the PSR and presents an internal critique. He argues that the set of all contingent facts—the BCCF—must have an explanation, since the PSR prohibits brute facts. But, he contends, this explanation cannot be necessary, because (he claims) a necessary proposition cannot explain a contingent one. Hence, the explanation must itself be contingent. But that would mean the explanation of the BCCF is contained within the BCCF, making it self-explanatory. And that, van Inwagen concludes, is incoherent.
Let’s now examine why this conclusion is not as damning as it seems.
Part IV: Why Accept the PSR
As noted earlier, the Principle of Sufficient Reason went largely unquestioned for nearly two thousand years. Ancient metaphysicians recognized its foundational role in the intelligibility of reality and wove it deeply into their systems—handing down this truth to later thinkers, many of whom inherited it without fully grasping its original necessity.
The PSR is central to the question of being—namely, what is and what is not. According to Parmenides, the father of Western philosophy, what is not is totally unknowable, since to know is to understand. Therefore, the object of thought is always being, and non-being is unintelligible. This implies something central to metaphysics: thought and being are co-extensive. As Dr. Eric Perl puts it in Thinking Being (p. 14), being is everything available to thought. If that’s the case, then reality as a structured whole is intelligible.
From this, the PSR follows necessarily. Without it, something would remain unintelligible—a brute fact—and thus unavailable to the intellect. But if being is everything that can be thought, then nothing can lie outside the PSR's scope.
Why is something rendered unintelligible when it lacks an explanation? It’s simple. As I’ve discussed in another post, being is completely self-possessed—that is, everything that it is. This is the principle of identity, without which thought itself collapses. If something is not everything that it is, it could be both itself and not itself—a violation of the principle of non-contradiction.
If a thing does not possess the nature by which it is, there is nothing to ground its intelligibility. It becomes opaque to reason—something the intellect can perceive but not comprehend. This is what we mean by a brute fact: not merely something unexplained, but inexplicable in principle. Such a thing leaves the intellect in suspension. Intelligibility requires that a thing have that in virtue of which it is—a cause, ground, or essence. Otherwise, understanding is impossible.
In affirming the intelligibility of the world, we must affirm the PSR.
Part V: How To Address the Problem
Now we face the supposed contradiction: if the PSR is necessary for thought, how can it also be incoherent? If it isn’t incoherent, where does van Inwagen’s argument go wrong?
The most obvious premise to challenge is (5)—that a necessary proposition cannot explain a contingent one. At first, this seems manifestly false. But upon closer inspection, it’s not so easily dismissed.
Van Inwagen, alongside Graham Oppy, argues that if a necessary being causes a contingent state of affairs, then the link itself would have to be necessary—otherwise, the contingent state wouldn’t follow. But if the link is necessary, then the outcome is necessary too. Thus, what we thought was contingent turns out not to be. This collapses contingency into necessity, contradicting the appearance of genuine contingency in our experience. And, honestly, I think this argument succeeds. It’s controversial, no doubt, but I haven’t seen a convincing defeater yet.
That said, we’re left with a metaphysical choice:
Accept the PSR and embrace necessitarianism, or
Deny the PSR and render some portion of reality unintelligible.
For me, the choice is clear. The latter option is far more troubling. Indeed, I would call it a non-issue, since Platonism has always affirmed that the One necessarily overflows into reality. The Good is self-diffusive. Reality, therefore, is necessary, not in a deterministic sense, but in the metaphysical sense of being the necessary expression of fullness.
Some might object that this sacrifices the intuitive appeal of contingency. But that’s not persuasive to me. Philosophy demands that we follow the argument where it leads, even when it challenges common sense. And besides, what is common sense for one age may be nonsense for another.
Final Thoughts
This conclusion may strike some as anticlimactic, but that’s the nature of philosophy. Arguments are refined by objections, and clarity arises only when we see the consequences of our metaphysical commitments. I’m not breaking new ground, I’m returning to the ancient ground. Aristotle, Plotinus, and the Neoplatonists saw the world as a necessary procession from the divine. The One’s return to itself is so complete, so overflowing with richness, that it necessarily gives rise to all things. The world, in this sense, is not an accident but a revelation.
And perhaps that is what makes it so beautiful. We ought not linger on what “could have been,” for there is nothing but what is. To affirm the PSR is to affirm that what is, is intelligible, and that is the first act of philosophy.
Very interesting article. Peter van Inwagen's argument vaguely reminded me of Russell's paradox. This definitely inspired me to learn more on this subject!