Does Alexander Pruss succeed in establishing a modified sufficient reason principle (modal PSR) capable of avoiding classical objections while preserving the cosmological argument?

This question touches upon one of the most ambitious philosophical projects in contemporary philosophy of religion: Alexander Pruss's attempt to reformulate the Principle of Sufficient Reason (PSR) in a way that preserves its explanatory power while avoiding its classical problems. Pruss's project, developed primarily in "The Principle of Sufficient Reason: A Reassessment" (2006), represents a turning point in the debate over cosmological arguments.

Inadequate Responses to Avoid

From some defenders of cosmological arguments:

"Pruss has definitively rescued PSR from all objections." An exaggeration. Even Pruss himself acknowledges that his formulation doesn't solve all problems, but rather reduces them. Claiming a "final solution" misses the precision of the philosophical project.

"Modal PSR is as clear and intuitive as classical PSR." Inaccurate. Modal PSR is more technically complex, requiring understanding of modal logic and contemporary metaphysics. Intuitive simplicity is not among its features.

"Anyone who rejects modal PSR rejects rationality itself." A methodological error. Many contemporary philosophers (van Inwagen, Bennett) reject even modified versions of PSR without abandoning rationality. The debate is deeper than mere "acceptance/rejection of rationality."

From some critics:

"Pruss's project is merely word-play to save an old argument." An unhelpful oversimplification. Pruss provides precise metaphysical distinctions, published in top academic journals. Rejecting the project without engaging its details is not criticism.

"Van Inwagen proved the impossibility of any form of PSR." Incorrect. Van Inwagen's criticism primarily targets classical PSR. Pruss develops detailed responses to this criticism, and the debate continues.

"Quantum physics refutes any causal principle." An unjustified leap. Different interpretations of quantum mechanics treat causality in different ways. Pruss discusses how modal PSR is compatible with most standard interpretations.

Why These Responses Are Inadequate

They share a failure to grasp the technical nature of Pruss's project. Modal PSR is not merely a "verbal modification," but a metaphysical restructuring that benefits from developments in modal logic and analytic metaphysics.

Classical Problems with PSR

The classical Principle of Sufficient Reason (Leibniz, Spinoza) states: "For every truth/fact, there is a sufficient explanation for why it is so and not otherwise." The main problems:

The Problem of Cosmic Necessity (Modal Collapse): If everything has a sufficient reason, and if sufficient reason entails its consequent, then everything is necessary. This negates possibility and free choice. (Spinoza accepted this consequence, but most philosophers reject it.)

Van Inwagen's Problem: Suppose PSR is true. Let's ask: "Why do these particular possible things exist, rather than other possible things?" Any answer will be either circular (presupposing the existence of some possibles) or make everything necessary.

The Problem of Negative Facts: What is the sufficient reason for the non-existence of unicorns? Classical PSR seems to require finding "causes" for non-existence, needlessly multiplying ontology.

The Problem of Brute Simplicity: If every explanation needs explanation, we get either infinite regress or circularity. Even if we assume a "necessary being" as ultimate explanation, the question remains: why this necessary being with its specific properties, rather than another necessary being?

Pruss's Formulation: Modal PSR

Pruss proposes a modal version of PSR:

Modal PSR: "For every possible fact p, there exists a possible world w where p has an explanation in w."

Or in another formulation: "◇(p) → ◇(p & ∃q(q explains p))"

The crucial difference: Classical PSR says "every fact has an actual explanation," while modal PSR says "for every possible fact, it's possible that it has an explanation." This allows for facts without explanation in our world, but prevents the existence of facts that are "in principle unexplainable."

How Modal PSR Avoids Classical Problems

Avoiding Cosmic Necessity: Modal PSR doesn't say every fact has an actual explanation that entails it. It only says it's "possible" for it to have an explanation. This allows for genuine contingencies: a contingent fact q can lack explanation in our world, but have explanation in another possible world.

Example: A free choice between A and B. In our world, the person chose A without a determining sufficient reason. But in another possible world, the same person in the same circumstances has a reason for choosing A. Modal PSR is preserved without negating freedom.

Response to Van Inwagen: The question "Why these particular possibles?" presupposes classical PSR. Under modal PSR, the set of existing possibles can be contingent without explanation in our world, but have explanation in another world. No need to make everything necessary.

Negative Facts: Modal PSR doesn't require actual explanation for every non-existence. It suffices that explanation is "possible." In some possible worlds, there may be explanation for unicorns' non-existence (natural laws preventing them), without needing to inflate our world's ontology.

Simplicity and Ultimate Explanation: Pruss accepts the existence of "brute facts" in our world, but denies the existence of facts that are "necessarily brute." Every fact, however brute it appears, has a possible explanation in some world.

How Modal PSR Preserves the Cosmological Argument

Despite modal PSR being weaker than the classical formulation, Pruss argues it suffices for the cosmological argument:

Step One: Consider the totality of contingent beings (BCCF - Big Conjunctive Contingent Fact). This is a possible fact.

Step Two: By modal PSR, it's possible that BCCF has an explanation. That is: ◇(∃x(x explains BCCF)).

Step Three: Any explanation of BCCF cannot itself be contingent (otherwise it would be part of BCCF, and nothing can explain itself). Therefore, the possible explanation must be necessary.

Step Four: Using the S5 principle in modal logic (◇□p → □p), if it's possible that a necessary being exists, then it exists necessarily.

Conclusion: There exists a necessary being that explains (in some possible worlds) the totality of contingents.

Criticisms of Modal PSR

The Criticism of Excessive Simplicity (Bennett, Della Rocca): Even modal PSR is too strong. Why must every fact have a "possible" explanation? Perhaps some facts are necessarily brute, inexplicable even in any possible world.

Pruss's Response: This makes rationality fundamentally limited. If we accept facts that are "in principle unexplainable," we open the door to rejecting any search for explanation. Modal PSR preserves hope for understanding without imposing actual understanding.

The Modal Obscurity Criticism (Oppy): What does "explanation in a possible world" mean? Do explanations exist across worlds? How does something in world w1 explain a fact in world w2?

Pruss's Response: Explanation is not "trans-world." The meaning is: in a possible world w, there exists a version of fact p and it has explanation in the same world w. This suffices to preserve the intuition that facts are "intelligible."

The Inadequacy for Cosmological Argument Criticism (Koons): Even if modal PSR succeeds in establishing a necessary existent, the leap from "possible explanation" to "actual explanation" needs additional justification. The argument only proves it's "possible" that the necessary being explains the world, not that it actually does.

Pruss's Response: He develops additional arguments (from simplicity, theoretical elegance) to favor that the possible explanation is actually realized. But this weakens the argument compared to classical formulations.

Contemporary Developments (2015-2026)

The "defense of modified PSR" current includes Pruss, Joshua Rasmussen, and Robert Koons. They develop different formulations: PSR for types, probabilistic PSR, partial PSR. All attempt to balance explanatory power with avoiding problems.

The "radical PSR criticism" current includes Graham Oppy and John Mackie (in earlier works), arguing that any PSR version strong enough for cosmological arguments will reproduce the problems of the classical version.

Where We Stand in This Debate Today

Between 2020 and 2026, the debate over modal PSR witnessed notable developments. Joshua Rasmussen and Timothy Perrine developed "probabilistic" versions of PSR that avoid full modal commitment, while Oppy continued his criticism, asserting that any formulation sufficient for the cosmological argument will reproduce the problems of the classical formulation. Pruss himself expanded his project in later works, responding to the modal obscurity criticism with finer distinctions between "explicability" and "existence of actual explanation." The general trend in the literature indicates growing recognition that modal PSR represents genuine progress over the classical formulation, but the dispute now centers on whether it alone suffices to establish the cosmological argument, or whether it needs supporting premises. The debate hasn't been settled, but its structure has become more precise and technical than it was two decades ago.

From the Perspective of Rational Weighing (rajḥān ʿaqlī)

Pruss's project aligns structurally with the method of cumulative rational weighing in several respects:

─ Modal PSR doesn't claim demonstrative certainty, but establishes rational probability: saying that facts are in principle explicable is more probable than saying there exist facts that are "necessarily opaque" and cannot be understood in any possible context.

─ The cosmological argument built on modal PSR doesn't work in isolation, but gains strength within cumulative argumentation that includes fine-tuning arguments, moral arguments, and others. Alone, it establishes the possibility of a necessary explanatory being; together with others, it raises the degree of probability.

─ In fairness to criticism: The objection that modal PSR is too weak to produce actual explanation is a serious one. But within the framework of rational weighing, the relative weakness of one premise doesn't negate its contribution to overall probability if it combines with independent evidence.