Does Peter van Inwagen's critique of the Principle of Sufficient Reason succeed in showing that it entails absolute determinism, or can the principle be formulated in an immunized manner?

This question touches one of the deepest tensions in analytic philosophy of religion: the relationship between the Principle of Sufficient Reason (PSR) and human freedom. Peter van Inwagen—one of the most prominent contemporary analytic philosophers of religion—presented in "An Essay on Free Will" (1983) and "Metaphysics" (4th ed., 2018) an influential critique of PSR, claiming that it entails absolute determinism and negates libertarian freedom. This critique places pressure on cosmological arguments that rely on PSR (especially Leibniz's argument), and calls for methodical responses from defenders of PSR.

Inadequate Responses to Avoid

From some defenders of PSR:

"Van Inwagen is just a determinist who wants to deny God's existence." Useless ad hominem. Van Inwagen is a practicing Christian who believes in God, but rejects PSR for philosophical reasons. His critique stems from his commitment to libertarian freedom, not from atheism.

"PSR is clearly self-evident; whoever denies it denies reason." Misleading oversimplification. PSR has multiple formulations, some stronger than others. Van Inwagen does not deny that things have causes, but denies that everything has a sufficient reason that necessarily entails it. The difference is subtle but decisive.

"PSR and freedom can be easily reconciled." Bypassing the real difficulty. The tension between PSR and libertarian freedom is philosophically deep, and resolving it requires precise technical work, not merely asserting compatibility.

From some critics of PSR:

"Van Inwagen proved that PSR is false, end of story." Overestimating the critique's force. Van Inwagen presented a strong argument, but defenders of PSR have developed serious responses. The debate continues and has not been settled.

"Any PSR necessarily entails determinism." Inaccurate generalization. There are weak formulations of PSR that may not entail determinism. The debate concerns which formulation is sufficient for cosmological arguments and consistent with freedom.

Why These Responses Are Inadequate

They share a failure to grasp the technical complexity of the debate. The question is not "Is PSR correct or false?" but "Which formulation of PSR is philosophically adequate and consistent with freedom?" This requires precise analysis of different formulations and their implications.

Structure of Van Inwagen's Argument Against PSR

Van Inwagen formulates strong PSR as follows: "For every fact p, there is a sufficient explanation for why p and not ¬p." This includes:
- Necessary facts (explained by their necessity)
- Contingent facts (explained by sufficient causes)
- Acts of free will (?)

His central argument:

First Step: PSR entails that every action has a sufficient explanation.

If I chose A instead of B, according to PSR, there is a sufficient explanation for why I chose A. This explanation is either:
- Internal (my desires, beliefs, nature)
- External (circumstances, influences, divine decree)
- A mixture of both

Second Step: Sufficient explanation entails necessity.

If the explanation is truly "sufficient," then it makes the outcome necessary. If it were possible for me to choose B despite the same sufficient explanation for choosing A, then the explanation is not "sufficient" in the sense required by PSR.

Third Step: Necessity negates libertarian freedom.

Libertarian freedom requires "genuine alternative possibilities" (PAP). At the moment of choice, it must be genuinely possible for me to choose either A or B. But if there is a sufficient explanation that entails A, then B is not genuinely possible.

Fourth Step: PSR entails universal determinism.

If PSR is applied to all events, then every event is necessarily determined by its sufficient causes. The causal chain extends to the infinite past or to a necessary first cause. In both cases, everything that happens now is necessarily determined.

Van Inwagen's Conclusion

Strong PSR is inconsistent with libertarian freedom. Since we have strong reasons to believe in freedom (direct experience, moral responsibility), we must reject strong PSR.

Responses from PSR Defenders

Three main lines of defense have developed:

First Defense: Restricted PSR

Alexander Pruss and Timothy O'Connor developed "restricted" formulations of PSR:
- PSR-Scope: applies only to natural events, not to acts of free will
- PSR-Foundation: everything contingent has a foundation in something necessary, without complete determination
- PSR-Tendency: causes tend toward outcomes without necessarily entailing them

These formulations attempt to preserve sufficient explanatory power for cosmological arguments without entailing determinism.

Van Inwagen responds: These formulations are too weak for cosmological arguments. If some things (free actions) do not need sufficient explanation, why does the universe?

Second Defense: Redefining "Sufficient Explanation"

Robert Koons and Joshua Rasmussen in "The Cosmological Argument from Contingency" (2018) propose a distinction:
- Entailing sufficient explanation: necessarily entails the outcome
- Non-entailing sufficient explanation: makes the outcome intelligible without entailing it

Free actions have sufficient explanation of the second type: the agent, with their capacities and desires, explains the action without entailing it.

Van Inwagen responds: This empties "sufficiency" of its meaning. A "non-entailing" explanation is not "sufficient" in the sense required by traditional PSR.

Third Defense: The Molinist Solution

William Lane Craig and Thomas Flint use "Middle Knowledge":
- God knows what every free agent will choose in every possible circumstance
- This knowledge does not cause the choices, but reflects them
- PSR is preserved: every event has an explanation (agent's will + God's knowledge)
- Freedom is preserved: the agent could have chosen otherwise

Van Inwagen responds: Molinism faces the "Grounding Problem." What makes counterfactual conditionals true? If there is a ground, we return to determinism. If not, then PSR is violated.

Pruss's Critique from a Different Angle

Alexander Pruss in "The Principle of Sufficient Reason: A Reassessment" (Cambridge UP, 2006) argues that van Inwagen confuses two levels:
- Metaphysical level: Do things have causes?
- Epistemic level: Can we know all causes?

PSR can be metaphysically correct even if some explanations (for free actions) are not fully knowable from a human perspective.

Proposed Immunized Formulation

Some contemporary philosophers (Jonathan Kvanzig, Michael Della Rocca) propose:

"PSR-Proportionality: For every contingent fact, there is an explanation proportionate to its nature."
- Physical events: entailing causal explanation
- Mental events: teleological/intentional explanation
- Free actions: agent-causal explanation
- Existence of the universe: explanation by metaphysical necessity or necessary agent

This preserves explanatory power without imposing a single model of explanation on all phenomena.

A Deeper Problem: God and PSR

Van Inwagen raises: Even if we solve the problem of human freedom, the problem of divine freedom remains. If God is free, and PSR is correct, why did God create this world and not another?
- If there is a sufficient explanation, then God is compelled
- If not, then PSR is violated

Defenders respond: God's actions are explained by His necessary nature (goodness, wisdom) without entailing specific actions. God is free to choose how to realize goodness.

From the Angle of the Cosmological Argument

The crucial question: Do cosmological arguments need strong PSR?

Some formulations (Koons, Pruss) are satisfied with a "weak explanatory principle": contingent things need some explanation, even if not entailing. This may suffice to reach a necessary first cause.

Van Inwagen partially accepts this but insists: Even weak PSR faces problems with freedom and quantum randomness.

The Deeper Philosophical Point

The debate reveals a fundamental tension in metaphysics between explanatory completeness and genuine agency. This tension may be irreducible, forcing us to choose our metaphysical priorities.

Where We Stand in This Debate Today

The period 2020-2026 witnessed tangible developments. Rasmussen in "Necessary Existence" and subsequent works developed formulations of PSR that avoid strict entailment, contenting themselves with a "principle of metaphysical explanation" that demands an existential foundation for every contingent thing without imposing causal determinism. Della Rocca continued defending strong PSR within his Spinozan framework, acknowledging its deterministic price but considering it preferable to the alternative. Pruss and Leuenberger (2022) proposed probabilistic formulations: causes make outcomes probable without entailing them, which some called "Dispositional PSR." Van Inwagen has not retreated from his basic position but acknowledged (2021) that weak formulations are harder to critique. The general trend today leans toward distinguishing levels of explanation instead of the binary "complete PSR or nothing," which represents real progress in the debate even if it has not settled it.

From the Angle of Rational Preference (rajḥān ʿaqlī)

Van Inwagen's critique represents a precise test for the methodology of cumulative rational preference:
─ Van Inwagen is correct that strong entailing PSR generates genuine tension with freedom. This is a price that must be acknowledged.
─ However, modified formulations (proportional, dispositional, non-entailing) retain the explanatory core required for cosmological arguments without complete deterministic slide.
─ Rejecting every form of PSR pays a higher price: accepting contingent facts with no explanation whatsoever, which undermines rationality itself more than the tension with freedom undermines it.
─ Within cumulative argumentation, PSR is not required to be certain, but to be more probable than its negation. The probability here combines with evidence from fine-tuning, consciousness, and morality to form a composite indication favoring a necessary foundation that explains contingent existence.
─ Conclusion: Modified PSR is rationally probable, not certain, and sufficient to carry the cosmological argument within a cumulative structure.