Onus probandi means “burden of proof,” and in philosophy it communicates the idea referenced above; namely, that entities making positive claims are required to bring forth arguments and data in support of their claim. Those denying such claims aren’t required to do anything until some positive proof lies on the table.
Last month, I participated in a Protestant & Roman Catholic dialogue about the Reformation at a nearby Christian university. The experience has left me reflecting on the fundamental issues that continue to divide Protestants and Roman Catholics, one of which is the authority of Scripture vis-à-vis tradition and living ecclesiastical authorities (the magisterium). As Protestants we maintain that Scripture alone constitutes God’s inspired, infallible Word, and, without denying the legitimacy of subordinate authorities (creeds, confessions, church councils, general assemblies, etc.), we nevertheless deny the status of such subordinate authorities and their proclamations as divine (and therefore infallible) Word.
A fairly common rejoinder to a Protestant articulation of sola Scriptura is: “where does Scripture teach that?” Roman Catholic apologists love to ask Protestants to demonstrate sola Scriptura from Scripture, and — if and when they struggle to do so — suggest that Protestants either cannot prove this basic article of their faith from their own acknowledged infallible and authoritative text (at which point the article crumbles), or that they must appeal to some extra-Scriptural authority to defend the claim of Scripture’s sole authority, thereby rendering the principle of Scripture’s exclusive authority self-defeating de facto. The demand to prove sola Scriptura from Scripture, in other words, is intended to leave Protestants tongue-tied and thereby receptive to arguments for infallible authorities above and beyond the biblical text.
As an apologetic strategy, asking Protestants to prove sola Scriptura from Scripture may be effective. But it’s nevertheless devious, because it violates one of the most basic principles of logic, which is that positive affirmations, not denials, require proof.
If, I would argue, Protestants are too effectively maintain their position on sola Scriptura moving forward, they might do well to buttress it with familiarity and efficiency with another Latin phrase, onus probandi, and what that Latin phrase entails in the realm of epistemology.