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.
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. So, for instance, if I claim that the Lochness Monster actually exists (which, I think we can all agree, she does), the onus probandi rests on me to demonstrate such. If I respond to your denial of Nessie with "prove that she doesn't exist!", I've not won the argument or validated my claim, even if I left you perplexed about how to continue the conversation. Likewise, if I claim that Chinese fortune cookies are a medium of divine communication, the burden of proof rests on me to make my case. Merely insisting that you prove otherwise and then sitting back with a smile on my face as you fail to demonstrate the un-divine provenance of fortune cookies is bad form to say the least.
But this is essentially what Roman Catholic apologists do when they insist that Protestants prove sola Scriptura from Scripture. After all, Protestants and Roman Catholics agree that Scripture is "breathed out by God and profitable for doctrine, for reproof, etc." (1 Tim. 3:16). They agree that Apostolic written testimony regarding Christ's person and work is "the Word of God," not "the word of men" (1 Thess. 2:13). Protestantism's claims regarding the existence of a divine Word from God stop there (and so remain far more modest than Rome's claims). To put the matter another way, Protestants can sound a hearty "amen!" to the Council of Trent's claim that the "written books" of Scripture constitute a fountain of "saving truth and moral discipline" (Fourth Session). It's Trent's further claim that "tradition" equally constitutes a fountain of saving truth and moral discipline that gives Protestants pause, not to mention the claims eventually made by Rome (at the First Vatican Council) for the infallibility of the magisterium when it adjudicates theological issues.
Rome essentially claims the same status for tradition and magisterium as it does for Scripture. It claims, that is, that tradition and magisterium belong to the category of "Word of God" rather than "word of men." Whether true or false, the onus probandi rests entirely on Rome to validate such claims. In my experience, proofs proffered in defense of Rome's claims regarding tradition and magisterium fall rather short. More often than not, defenders of Rome's claims simply seek to shirk the onus probandi for their position, and/or illegitimately transfer it to Rome's detractors.
In sum, sola Scriptura, Protestants would do well to remember, is only a positive claim insofar as it posits the inspiration and infallibility of Scripture. The onus probandi for that positive claim does indeed rest on us. It falls to us, in other words, to defend our positive claims about Scripture. But in dialogues with our Roman Catholic friends, a defense of Scripture's status as "breathed out by God" should be rather easy since that claim constitutes common ground. In all other regards, sola Scriptura constitutes the rejection of claims advanced by others -- claims for the inspired and infallible status of some extra-Scriptural word (whether of the Mormon, Roman Catholic, Pentecostal, or vanilla evangelical variety). The onus probandi for those claims rests on others. Until convincing proof for the inspired and infallible status of the Book of Mormon, tradition, the magisterium, fortune cookies, or any other proposed medium of divine communication forth comes, we can and must stand our ground, so help us God.