Principle of sufficient reason

The philosophical principle that every contingent fact has a sufficient reason why it obtains rather than not.

Version

v1 · 2026-09-28 · History

Domain-specific #

11484

Domain group

Humanities

Origin domain

Philosophy

Subdomain

Metaphysics and Explanation → Philosophy

Aliases

PSR, Leibnizian principle of sufficient reason

Core Idea

The principle of sufficient reason is a philosophical thesis about explanatory completeness. In a common formulation, every contingent fact has an adequate reason why it obtains rather than not or rather than a relevant alternative. The reason need not be known to us, and the principle does not by itself identify a particular cause. Leibniz presents it beside the principle of contradiction in the Monadology: the latter concerns the impossibility of contradictions, whereas sufficient reason addresses why facts and true assertions are thus rather than otherwise. Disagreement over exactly which facts or propositions fall within the demand is part of the doctrine's history, not a license to conflate every version.

A reason can be logical, metaphysical, or causal depending on the formulation. Necessary truths may be explained by their necessity; the contingent-fact form especially presses the question of why one possible state is actual. A universal physical-causation rule is stronger and narrower in explanatory kind. Leibniz's challenge to absolute spatial placement in the Clarke correspondence shows the principle at work: if two placements are wholly indiscernible, the alleged choice seems to lack a differentiating ground. A critic can reject the demand, revise the account of space, or defend a different account of choice. PSR supplies an explanatory obligation under a philosophical commitment; it is not an observation that every human inquiry has already succeeded.

Structural Signature

Sig role-phrases:

• Contingent explanandum — Identifies a fact, existent, event, or true proposition whose obtaining could have been otherwise under the chosen formulation. It is constitutive. Counterfactual: A necessary truth may have a reason in necessity but is not the characteristic contingent-fact test.
• Contrastive alternative — States the rather-than-not or rather-than-otherwise question to which a sufficient reason responds. It is constitutive. Counterfactual: A merely associated condition with no explanation of this contrast is not yet sufficient reason.
• Explanatory sufficiency claim — Asserts that some adequate reason exists for every in-scope explanandum, not merely that one possible influence can be named. It is constitutive. Counterfactual: A brute contingent fact admitted to have no explanation contradicts this formulation.
• Reason–cause distinction — Allows logical, metaphysical, and causal explanatory modes according to the version defended. It is boundary. Counterfactual: Replacing every reason with a temporally prior efficient cause silently adopts a stronger thesis.
• Epistemic access limit — Separates existence of a sufficient reason from an individual's ability to identify it. It is boundary. Counterfactual: An unexplained observation is not by itself a counterexample until the claim that no reason exists is justified.

What It Is Not

• Not the question why. Asking for an explanation does not assert that one must exist for every contingent fact.
• Not universal efficient causation. A sufficient reason may be noncausal under a defended formulation.
• Not guaranteed discovery. The thesis concerns existence of a reason, not human access to it.
• Not a tautology. A brute contingent fact would challenge the principle rather than satisfy it by definition.
• Closest near-miss. A general research norm of continuing to seek explanations is the closest miss: it guides inquiry without asserting that every contingent fact actually has a sufficient reason.

Scope of Application

• Metaphysical explanation. Assess whether contingent facts may terminate in brute unexplained facts.
• Leibnizian argument. Test whether an alleged choice among equivalent alternatives has a differentiating ground.
• Philosophy of religion. Distinguish PSR premises in cosmological arguments from their further theological conclusions.
• Methodological caution. Separate a commitment to explanatory adequacy from a successful empirical causal account.

Clarity

For a contingent fact, ask what makes it obtain rather than a relevant alternative. PSR asserts that an adequate reason exists even if no one knows it. A policy of looking for explanations is the closest weak neighbor, and universal physical causation is a stronger neighbor that wrongly restricts the explanatory kind. A truly brute contingent fact, if admitted, is a counterexample to this common formulation rather than merely an unanswered question.

Manages Complexity

The short maxim hides several independent choices: the domain of facts, the strength of 'sufficient,' the alternative explained, whether reasons are causes, and whether unknown reasons count. Keeping these choices visible explains why Leibniz's absolute-space argument and a modern brute-fact objection can disagree without simply using different words. The principle organizes a dispute; it does not settle every explanatory chain by itself.

Abstract Reasoning

1. Declare whether the version quantifies over contingent facts, all truths, existents, or events.
2. State the focal fact and a relevant alternative to be explained.
3. Ask what would count as an adequate reason in the chosen explanatory mode.
4. Distinguish no known reason from a claim that no sufficient reason exists.
5. Test a proposed brute fact or symmetry case without treating a causal story as the only possible response.

Knowledge Transfer

PSR can be applied from a dispute about cosmic existence to Leibniz's absolute-space placement argument when the explanandum, contrast, and sufficiency standard are restated. The latter does not transfer its spatial symmetry premise to every event. Scientific causal investigation may be motivated by explanatory ambition, but its empirical success or failure does not automatically establish or refute the universal metaphysical thesis.

Examples

Canonical

In Monadology §32, Leibniz asks why a real fact or true assertion is so rather than otherwise and holds that there must be a sufficient reason even when people do not know it. This is a direct statement of PSR's existence claim, not an empirical recipe for finding the reason and not a proof that every reason is an event cause.

Mapped back: Contingent explanandum → a real contingent fact or true contingent assertion; Contrastive alternative → why it is thus rather than otherwise; Explanatory sufficiency claim → a sufficient reason must exist; Reason–cause distinction → the passage says reason, not only prior physical cause; Epistemic access limit → reasons may be unknown to us.

Applied / In Practice

In the published Leibniz–Clarke exchange, Leibniz applies PSR against absolute-space placement: if positions were genuinely indistinguishable in all relevant respects, what sufficient reason could there be for creating the material world here rather than at a displaced position? The argument uses the explanatory demand to challenge the spatial picture. It is a documented philosophical application, not a settled physical experiment or a proof that Clarke accepted Leibniz's conclusion.

Mapped back: Contingent explanandum → the supposed placement of the material world in absolute space; Contrastive alternative → an otherwise equivalent displaced placement; Explanatory sufficiency claim → a differentiating reason is demanded for this placement; Reason–cause distinction → metaphysical ground of choice, not a measured mechanical cause; Epistemic access limit → the exchange debates the principle's application rather than locating an observed mechanism.

Structural Tensions

T1 — Universal Explanatory Demand versus Possible Brute Contingencies. The principle rules out genuinely reasonless in-scope facts, whereas a critic may take some contingent fact as explanatory terminus.

Diagnostic: Is the proposed terminus necessary, or is it a brute contingent exception?

T2 — Reason Exists versus Reason Is Knowable. The metaphysical thesis can be asserted despite human ignorance, but that separation reduces direct empirical testability.

Diagnostic: Has lack of current knowledge been mistaken for lack of any reason?

Structural–Framed Character

PSR is mixed but framed-leaning on the structural–framed spectrum: its quantifier and contrastive demand are formal, while the acceptance of universal explanatory completeness is a philosophical commitment. Evaluative weight: adequacy is assessed against a reason standard, not moral goodness. Human-practice-bound: contingent facts may occur without inquiry, but the thesis and the standards for an explanation are articulated in reasoning practices. Institutional origin: Leibniz's formulation is historically influential, not true by decree of a school. Vocabulary travels: reason, fact, and alternative are portable, but whether scientific causes, logical grounds, or metaphysical explanations qualify is contested. Import versus recognize: another philosophical claim that every in-scope contingency has adequate contrastive explanation may be PSR; a laboratory's pragmatic search for causes is not automatically the same universal thesis.

Its portable skeleton is an explanatory-demand relation across a declared class of facts. That thinner pattern could be a future-prime candidate, but no verified current prime is asserted as a strict parent: Necessity and Sufficiency analyzes conditional direction, and Causal Reasoning evaluates evidence for causal structure, neither universally asserts that every contingent fact has an explanation. Its character: a formally expressible but philosophically framed commitment about explanation, narrower than a general prime relation.

Structural Core vs. Domain Accent

The principle's quantifier gives it an abstract appearance, but its named identity retains a philosophical explanatory commitment.

What is skeletal. Given a fact, a contrastive alternative, and a standard of adequacy, one can ask whether a reason accounts for the fact's obtaining rather than the alternative. This explanatory-demand relation can be used in many inquiries and may warrant a future-prime candidate. Necessity and Sufficiency can test some implications within an answer but does not state that an answer must exist.

What is domain-bound. PSR strengthens that question into a general thesis over a declared domain of contingent facts. It must confront brute-fact objections, the difference between reason and cause, and the distinction between a reason's existence and our access to it. In the Clarke exchange Leibniz applies the demand to an alleged absolute-space symmetry; the spatial dispute is an application, not a definition of every PSR use. Dropping the universal explanatory claim leaves a useful research attitude but not the same principle.

Why this does not clear the prime bar. Other fields may seek explanations without endorsing PSR's universality or treating unknowable grounds as existent. A physicist can reject one causal model while continuing inquiry; that alone neither instantiates nor refutes the metaphysical thesis. The transferable question is thinner than the doctrine, while the named principle is contested under philosophy's choices about domain and adequate ground. Calling any search for a cause 'PSR' would import the name by analogy rather than recognize this exact commitment.

Instantiates / Related Primes

This entry is a kind of Law (Universal Principle).

• Related — necessity and sufficiency. A reason may include sufficient conditions, but the prime's implication-direction test does not assert that every contingent fact has a reason.

• Related — causal reasoning. Empirical cause assessment may answer some why-questions; PSR does not reduce all reasons to such causes.

• Related — causal adequacy principle. Descartes' claim about a cause containing at least as much reality as an effect is a different philosophical restriction.

Relationships to Other Abstractions

Current abstraction Principle of sufficient reason Domain-specific

Parents (1) — more general patterns this builds on

• Principle of sufficient reason is a kind of Law (Universal Principle) Prime

  Principle of sufficient reason is a strict kind of Law (Universal Principle): its frozen identity entails the parent's defining structure while adding domain-specific restrictions.

Children (1) — more specific cases that build on this

• Causal adequacy principle Domain-specific is a kind of, typical Principle of sufficient reason

  Causal adequacy is Descartes' reality-quantified version of demanding a sufficient ground for an effect: the cause must contain enough reality to explain it.

Hierarchy path (1) — routes to 1 parentless root

• Principle of sufficient reason → Law (Universal Principle) → Invariance

Neighborhood in Abstraction Space

Principle of sufficient reason sits in a moderately populated region (48^th percentile for distinctiveness): it has near-neighbors but no dense thicket of look-alikes.

Family — Unclustered & Miscellaneous (2551 abstractions)

Nearest neighbors

• Subverted Support — 0.89
• Hume's Fork — 0.89
• Trivial Objections — 0.86
• I'm Entitled to My Opinion — 0.86
• Moore's paradox — 0.85

Computed from structural-signature embeddings · 2026-10-08

Not to Be Confused With

• Universal causation. Tell: Does the thesis require an efficient physical cause or any adequate ground?
• Explanatory research norm. Tell: Is the claim that every contingent fact has a reason, or only that asking is useful?
• Known explanation. Tell: Is absence of discovery being mistaken for absence of any reason?
• Necessary truth. Tell: Is its ground being confused with a contingent event cause?

References

• Gottfried Wilhelm Leibniz, Monadology §§31–32, Hedge translation: https://en.wikisource.org/wiki/Monadology_%28Leibniz%2C_tr._Hedge%29
• Leibniz–Clarke papers, Leibniz's third paper (1716), absolute-space application: https://www.earlymoderntexts.com/assets/pdfs/leibniz1715.pdf
• Frozen Wikipedia discovery revision: https://en.wikipedia.org/wiki/Principle_of_sufficient_reason (revision 1360786118).
• Preserved source candidate: https://philopedia.org/arguments/leibnizs-contingency-argument/
• Preserved source candidate: http://plato.stanford.edu/entries/sufficient-reason/
• Preserved source candidate: https://www.jstor.org/stable/19854?seq=1
• Preserved source candidate: https://web.archive.org/web/20151117200947/http://www.rbjones.com/rbjpub/philos/classics/leibniz/monad.htm
• Preserved source candidate: http://www.banglajol.info/index.php/AFJ/article/download/12936/9300
• Preserved source candidate: http://www.gutenberg.org/ebooks/17147
• Preserved source candidate: https://archive.org/details/leibnizclarkecor00clar