The principle of sufficient reason collapses all contingency into necessity, so it should be rejected

The objection is serious, and it is often stronger than the quick online versions suggest. It is not merely the complaint that "everything has a cause" is false, since many defenders of Leibnizian cosmological arguments do not say that everything has a cause. They say, more carefully, that every contingent truth, or every contingent fact, has an explanation or sufficient reason. The modal-collapse objection asks whether that principle can be maintained without destroying the very distinction between what is contingent and what is necessary.

The issue matters because many cosmological arguments depend on some version of the principle of sufficient reason, or PSR. Leibniz, Samuel Clarke, and many contemporary philosophers sympathetic to cosmological reasoning hold that contingent reality cannot be an unexplained brute fact. Critics of unrestricted PSR are not all atheists: William Rowe and Graham Oppy have pressed versions of the worry from broadly atheistic or naturalistic perspectives, while Peter van Inwagen, a theist, has also argued that very strong explanatory principles threaten contingency and freedom. Contemporary defenders, including Alexander Pruss, Robert Koons, and Josh Rasmussen, have offered refined versions of the PSR and replies to collapse worries. So the honest question is not whether a crude slogan can be mocked. It is whether a strong PSR, of the kind that can support a Leibnizian argument, is internally stable.

The argument steelmanned

Here is the objection in its strongest form.

Let C be what Rowe-style discussions often call the big conjunctive contingent fact, or, in propositional terms, the conjunction of all actual contingent truths. C includes every actual truth that could have been otherwise: that this universe exists, that these laws obtain, that there are these particles, that certain persons exist, that a particular free choice is made, and so on. The critic grants, for the sake of the argument, that such a totality is a legitimate object of explanation. If the PSR is unrestricted, then C too must have a sufficient reason or explanation.

Now ask what kind of truth explains C. The explanation must be either necessary or contingent.

• If the explanation is necessary, then, on a strong understanding of "sufficient reason", the explanation entails or necessitates what it explains. A sufficient reason is not merely something compatible with its explanandum. It explains why the explanandum obtains rather than not. If a necessary truth entails C in the relevant modal sense, then every possible world contains that necessary truth and, by entailment, contains C as well. C would therefore be necessary. But C was introduced as the conjunction of actual contingent truths. So contingency has collapsed into necessity.
• If the explanation is contingent, then it is already included within C, since C contains all actual contingent truths. A part of a totality can sometimes explain another part of that totality, so the point is not that internal explanation is always useless. The problem is that a contingent explainer inside C cannot, by itself, explain C as a whole without also accounting for its own inclusion in C. Otherwise the explanation is circular, incomplete, or explanatorily defective. It is like explaining the whole book by appeal to one of the sentences inside the book, when that sentence itself still belongs to the totality needing explanation.

So, the critic argues, the strong PSR faces a dilemma. If the explanation of all contingent truths is necessary, then all contingent truths become necessary. If the explanation is contingent, the explanation is circular. Either way, the PSR cannot do the work Leibnizian arguments require.

The same pressure appears in theistic discussions of divine freedom. Suppose, as classical theists typically do, that God exists necessarily and necessarily has the divine nature. If God's necessary nature alone is taken as the sufficient reason for creation, and if sufficient reasons entail their effects, then creation follows necessarily. God could not have refrained from creating, nor could God have created a different world. But many theists want to say that creation is free and contingent: God could have created, not created, or created otherwise. A necessitating PSR therefore threatens that claim unless the explanation includes a non-necessitating act of will, a restricted PSR, or some other account of how reasons explain without fixing a single outcome.

This is why the objection is sometimes described as a route from Leibniz to Spinoza. Spinoza held a form of necessitarianism: everything follows from the divine nature with necessity. Leibniz wanted both a sufficient reason for the actual world and genuine contingency. The modal-collapse objection says that he cannot have both, at least not with a strong enough PSR.

Where the objection has real force

The objection succeeds, as a deductive internal critique, against a particular package of commitments:

• an unrestricted PSR applying to every contingent truth or fact, including the total conjunction of contingent truths;
• the claim that a sufficient explanation must entail, necessitate, or guarantee what it explains;
• the assumption that the conjunction of all contingent truths is itself a legitimate contingent fact requiring explanation;
• the rejection of circular or self-involving explanations.

If all four are granted, the collapse argument is powerful. The conclusion follows naturally. A necessary proposition that entails C makes C necessary. A contingent proposition that explains C is already inside C. So the critic is not relying on a cheap trick. The argument identifies a genuine tension between unrestricted explanation and robust contingency.

It also exposes a weakness in some popular presentations of cosmological arguments. If someone says, "Every contingent thing must have a sufficient explanation, and that explanation is a necessary being", but then also says, "The necessary being freely chose among genuinely open alternatives", the details matter. What kind of explanation is being offered? Does it entail the actual world? If not, in what sense is it sufficient? If yes, in what sense was the world genuinely contingent?

A defender of the PSR cannot simply wave this away. The critic is asking for the exact modal strength of the explanatory relation. Without that clarification, the argument risks sliding between two meanings of "sufficient": strong enough to rule out brute contingency when convenient, but weak enough to preserve freedom when challenged.

The strongest reply

One of the strongest replies is to deny that every adequate explanation must be necessitating. This is not an ad hoc move. In ordinary and philosophical practice, many explanations do not entail what they explain. A person's reasons may explain an action without making the action inevitable. In indeterministic causal cases, an explanation may cite conditions that made an event likely or intelligible without guaranteeing it. Even deterministic causal explanations usually necessitate only relative to a package of laws and initial conditions, not by logic alone. A teleological explanation may say why an agent chose one end without proving that no alternative was possible.

On this view, the PSR need not say:

For every contingent truth p, there is some reason r such that r entails p.

It may instead say something weaker:

For every contingent truth p, there is an adequate explanation of why p obtains, though the explanation need not necessitate p.

This distinction matters. If a necessary being freely causes a contingent world, then the explanation of the world's existence may be grounded in that being's power, agency, and intention, without the world being logically or metaphysically forced by the being's nature. On this proposal, the necessary source explains the existence of contingent reality in an agent-causal way; it does not entail that this exact contingent reality had to exist. The unresolved question is whether such an explanation is sufficiently contrastive: does it explain why this world exists rather than no world or another world?

Defenders of this reply often appeal to agent causation or libertarian freedom. On that model, a free agent can be the source of an action without being determined by prior conditions to perform exactly that action. The action is not brute in the sense of being wholly unexplained, because it is explained by the agent, the agent's reasons, and the agent's purposes. But it is not necessary, because the agent could have done otherwise. Applied to divine creation, the thought is that a necessary creator can explain a contingent world without making the world necessary. The concession is important: this reply depends on a controversial theory of agency, so it will not persuade philosophers who think all adequate explanations must be deterministic or necessitating.

This reply blocks the simplest formal modal-collapse inference. From "N explains C" one may not infer "N entails C" unless explanation has already been defined as entailment or necessitation. If explanation can be non-necessitating, then a necessary explanation of contingent reality does not automatically make contingent reality necessary. The critic can still argue that the resulting PSR is too weak, but that is a further argument, not the original modal collapse.

The cost of that reply

Still, the reply has a cost. The critic can reasonably ask whether the weaker PSR is strong enough for the Leibnizian argument.

If the PSR only means that contingent facts have some intelligible connection to a source, but not one that entails or contrastively explains why this world rather than another obtains, then the critic may say: the deepest contingency has not been removed. Why this creative act? Why this world? Why not no world? If the answer is "because the agent freely chose it", the next question is whether that free choice itself has a sufficient reason. If it does, and the reason necessitates the choice, freedom is threatened. If it does not, then the PSR has been weakened at precisely the point where the critic expected it to apply.

This is the central pressure. A non-necessitating explanation can preserve contingency and freedom, but it may do so by accepting a kind of ultimate contingency in the free act itself. Some philosophers will regard that as perfectly acceptable: free choices are explained by agents, reasons, and purposes, but not determined by them. Others will regard it as a controlled exception to the PSR.

Another reply is to deny that there is such a thing as C, the conjunction of all contingent truths, or to deny that arbitrary conjunctions require explanations. Perhaps each contingent truth can have an explanation without there being a further fact consisting of their total conjunction. This avoids the big-conjunctive-fact argument. But again, there is a cost. Many Leibnizian arguments are attractive precisely because they ask for an explanation of contingent reality as a whole, not merely for each item within it. If the defender refuses to allow the totality as an object of explanation, the resulting cosmological argument becomes more modest.

A third reply is to accept necessitarianism. One could say that the modal collapse is not absurd. Spinoza took something like this path, and some philosophers are less troubled by necessitarian conclusions than others. But this is not the route most classical theists want, because it appears to compromise divine freedom and the contingency of creation. It also conflicts with the ordinary modal intuition that many facts could have been otherwise.

What the objection does and does not establish

The modal-collapse objection establishes something important: an unrestricted, necessitating PSR is very difficult to combine with genuine contingency. If every contingent truth must have a sufficient reason that entails it, and if the conjunction of all contingent truths is itself to be explained, then the collapse argument has real force. The defender of such a PSR owes a serious answer.

It also shows that appeals to divine freedom cannot be casual. If God's nature necessarily explains creation in a necessitating sense, then creation seems necessary. If creation is free in a libertarian sense, then the explanation of creation must be non-necessitating, or the PSR must be restricted, or some account must be given of how a sufficient reason can incline without determining.

But the objection does not establish that every version of the PSR is false. It does not refute weaker principles of explanation. It does not show that contingent reality is brute. It does not prove naturalism or atheism. At most, it shows that a certain strong form of explanatory rationalism generates modal collapse unless carefully qualified.

The most balanced conclusion is this: the modal-collapse objection is a powerful internal critique of strong Leibnizian PSR arguments, and it succeeds against versions that combine an unrestricted PSR with necessitating explanation and an explanatory demand on the total conjunction of contingent truths. It forces defenders to choose among options: accept necessitarianism, weaken the PSR, deny that total conjunctions require explanation, or defend a non-necessitating account of sufficient explanation. Each option has philosophical costs. The objection is therefore not a quick refutation of theism as such, but it is a serious constraint on how any PSR-based argument must be formulated.