Principle of Sufficient Reason

The Principle of Sufficient Reason (Latin principium rationis sufficientis; often abbreviated PSR) is the principle that nothing happens without a sufficient reason why it should be so rather than otherwise. The principle is most extensively developed by Gottfried Wilhelm Leibniz, for whom it is one of the two foundational principles of his metaphysics (alongside the Principle of Non-Contradiction). The principle has been continuously contested since its formulation and remains one of the central topics of modern metaphysics.

Definition

The principle holds that for every fact, event, or proposition that is the case, there is a sufficient reason why it is the case rather than not the case. The principle is therefore both an epistemological commitment (we are entitled to seek and expect explanations for what is) and a metaphysical commitment (the world is structured such that explanations of the relevant kind always exist).

Different versions of the principle have different strengths. The strongest version requires that every fact whatever has a sufficient reason. Weaker versions restrict the principle to particular kinds of facts (contingent facts; facts about the existence of things; facts about events) or particular kinds of reasons (causal reasons; logical reasons).

Origin

The principle has roots in earlier philosophy (Aristotle's principle that nothing happens without a cause; the medieval scholastic principle that ex nihilo nihil fit, nothing comes from nothing), but its developed form belongs to Leibniz. Leibniz invokes the principle throughout his work — in the Monadology (1714) it is one of the two fundamental principles of reasoning; in the Leibniz-Clarke correspondence (1715–1716) it is deployed extensively in arguments against Newtonian absolute space and time.

The core claim

The core claim has three parts.

The world is intelligible through and through. The PSR is the commitment that reality is structured such that genuine explanations of phenomena are always (in principle) available. The world is not a brute given of unexplained facts; it is a rationally ordered whole.

Sufficient reasons can be distinguished from mere causes. A sufficient reason explains why something is the case in a way that mere causation may not. The PSR is therefore not simply the principle of universal causation; it is the stronger principle that any contrast (this rather than that, here rather than there, now rather than then) admits of explanation.

The principle has metaphysical consequences. From the PSR, Leibniz derives substantial metaphysical conclusions: the existence of God (as the necessary being whose existence is its own sufficient reason and which provides the sufficient reason for the existence of contingent beings); the doctrine of pre-established harmony among monads; the rejection of absolute space and time; the doctrine that this is the best of all possible worlds.

Leibniz's deployments

Leibniz uses the PSR throughout his philosophy in characteristic ways.

The cosmological argument for God. The most famous application. If every contingent fact has a sufficient reason, and the set of all contingent facts is itself a contingent fact, then there must be a sufficient reason for the set as a whole — and this reason cannot itself be a contingent fact (which would generate a regress). The sufficient reason must therefore be a necessary being whose existence is its own sufficient reason. This is God.

The argument against absolute space. In the Leibniz-Clarke correspondence with Samuel Clarke (representing Newton), Leibniz argues that absolute space cannot exist. If space were absolute and homogeneous, there would be no sufficient reason why God should place the cosmos at one location rather than any other; the choice would be arbitrary. Since (by PSR) no arbitrary choice is consistent with God's wisdom, absolute space cannot exist; space is relational, the order of coexisting things rather than a container in which things are placed.

The argument for the best of all possible worlds. Among the infinite possible worlds God could have created, God chose the actual world. By PSR, there must be a sufficient reason for the choice. The only sufficient reason consistent with God's perfect wisdom and goodness is that the actual world is the best possible — the one that maximizes overall goodness when all factors are considered. The famously caricatured Leibnizian doctrine.

The Spinozist version

Baruch Spinoza deploys a version of the PSR even more uncompromisingly than Leibniz. For Spinoza, the principle holds without restriction: every fact whatever has a sufficient reason. The consequences are stark: there are no genuinely contingent facts (every fact is necessary in the sense that its sufficient reason makes it necessary); free will in the libertarian sense does not exist (every action has a sufficient reason that makes it the action that occurs); the entire structure of reality follows necessarily from the nature of the single infinite substance.

Michael Della Rocca's Spinoza (2008) presents Spinoza as the most consistent rationalist on the grounds that he takes the PSR to its uncompromising conclusion. The reading has been substantially influential and continues to organize contemporary engagement with both Leibniz and Spinoza on the PSR.

The Humean and Kantian challenges

Hume attacked the PSR as having no rational foundation. There is no contradiction in supposing that some events occur without sufficient reason; the principle is therefore not a necessary truth. Hume's broader empiricist critique — that we have no impression of necessary connection in causation, that even the principle of universal causation is not rationally established — substantially undermines the PSR.

Kant reframes the principle in transcendental terms. The principle does not hold of things in themselves; it holds as a structuring condition of possible experience. Within the bounds of experience, every event has a cause (the Second Analogy of Experience); beyond those bounds, traditional metaphysical applications of the PSR (especially the cosmological argument for God) overreach and fail to deliver knowledge.

Common confusions

The PSR is not the same as the principle of causation. The PSR is stronger: every contrast admits of explanation, not just every event has a cause. The principle applies to facts and states of affairs as well as to events; it requires explanation of why this rather than that, not just why anything at all.

The PSR is not the same as determinism. Determinism is the doctrine that every event is necessitated by prior events plus the laws of nature; the PSR allows for sufficient reasons that are not deterministic causes (logical reasons, reasons grounded in necessity of nature, reasons grounded in choice).

The strength of the PSR varies across formulations. Different philosophers have defended different versions, ranging from the unrestricted Spinozist version to more limited versions that apply only to certain kinds of facts. The contemporary literature carefully distinguishes among these versions.

Place in the wiki

The Principle of Sufficient Reason is a satellite of the Pillar concept Substance (which provides the metaphysical framework within which the PSR operates) and is closely related to Free Will (which the strong PSR rules out in its libertarian form).

Further reading

• Leibniz — the most extensive developer of the principle
• Spinoza — the most uncompromising application
• Rationalism — the tradition the principle organizes
• Hume — the major empiricist critic
• Kant — the Critical reframing of the principle
• Free Will — the concept the strong PSR rules out

Satellite of Substance. The central methodological commitment of Leibnizian rationalism.