No brute facts: The Principle of Sufficient Reason in ordinary thought.

The Principle of Sufficient Reason (PSR) has been an influential thesis since the earliest stages of western philosophy. According to a simple version of the PSR, for every fact, there must be an explanation of that fact. In the present research, we investigate whether people presuppose a PSR-like principle in ordinary judgment. Across five studies (N = 1121 in total, U.S., Prolific), we find that participants consistently make judgments that conform to the PSR. Such judgments predictably track the metaphysical aspects of explanation relevant to the PSR (Study 1) and diverge from related epistemic judgments about expected explanations (Study 2) and value judgments about desired explanations (Study 3). Moreover, we find participants' PSR-conforming judgments apply to a large set of facts that were sampled from random Wikipedia entries (Studies 4-5). Altogether, the present research suggests that a metaphysical presumption plays an important role in our explanatory inquiry, one that is distinct from the role of the epistemic and non-epistemic values that have been the focus of much recent work in cognitive psychology and philosophy of science.