The Principle of Sufficient Reason, often abbreviated as PSR, is a foundational metaphysical principle stating that for every fact or every contingent being, there exists a sufficient reason explaining why it is the case rather than otherwise. When deployed in the cosmological argument, the PSR generates a powerful inference: if every contingent fact has a sufficient reason, then either the totality of contingent facts has a sufficient reason that is itself non-contingent, or there is no explanation at all — and the PSR rules out the second option.
The principle finds its most influential formulation in the work of Gottfried Wilhelm Leibniz, particularly in the Monadology and the correspondence with Samuel Clarke. Leibniz held the PSR as one of the two great principles of reasoning, alongside the principle of non-contradiction. For Leibniz, the PSR applies to all truths, both necessary and contingent, though the kind of sufficient reason differs: necessary truths have their sufficient reason in logical demonstration, while contingent truths have theirs ultimately in the will of God choosing the best possible world. Contemporary analytic philosophers including Alexander Pruss and Robert Koons have defended modal versions of the PSR, arguing for variants that avoid the strongest objections while preserving the core inferential power needed for the cosmological argument.
The principle has attracted substantial criticism. Hume questioned why explanation must be required at all, suggesting that demand for sufficient reason might be a psychological habit rather than a metaphysical necessity. Peter van Inwagen argued that a strong PSR leads to modal collapse: if everything must have a sufficient reason and the reason is itself necessary, then everything becomes necessary, eliminating contingency entirely. Defenders respond with weaker formulations of the PSR (restricted to contingent facts, or to possible explanations rather than actual ones) designed to evade the modal collapse objection while retaining sufficient strength to drive the cosmological inference.
Among other formulations in the cosmological family, the Principle of Sufficient Reason is methodologically distinctive in that it operates as a general explanatory principle rather than an argument with specific empirical or historical premises. The Leibnizian Cosmological Argument applies the PSR to the contingent universe as a whole. The Contingency Argument can be understood as a particular instance of PSR reasoning. The First Cause and Thomistic versions rely on more specific causal principles rather than the general PSR. The Kalam version, by contrast, relies on a quite different premise concerning temporal origination.