What's the target of sola scriptura?

i) A Catholic trope is that sola scriptura is self-contradictory. Unless you can establish the canon of Scripture from Scripture alone, sola scriptura is false. The cliche formulation is to claim that unless the Bible contains its own table of contents, sola scriptura is false. Unless Scripture itself lists the books of Scripture, sola scriptura is false. 

It's striking to see the copycat mentality among Catholic apologists in that regard. They all repeat this trope as if it's self-evidently true. 

ii) I've never seen a Catholic apologist quote a classic Reformed or Lutheran creed which says sola scriptura excludes extrabiblical evidence for the canon. I've never seen a Catholic apologist quote a major 16-17 Reformed or Lutheran theologian who said sola scriptura excludes extrabiblical evidence for the canon. What historical documentation do Catholics have that sola scriptura was ever intended to exclude extrabiblical evidence for the canon? 

iii) Perhaps, though, the objection is that while sola scriptura doesn't rule out extrabiblical evidence for the canon by design, that's a necessary implication of the principle, and Protestants are simply inconsistent in that regard. They artificially exempt the canon of Scripture from sola scriptura. 

Problem is, I've never seen a Catholic apologist present an argument for why that's a necessary implication of sola scriptura. Rather, they just assert that to be the case. 

iv) What is the target of sola scriptura? Is the target extrabiblical evidence for the canon? No. The target of sola scriptura is a particular view of church authority. Sola scriptura stands in opposition to the view that the Roman Magisterium is God's oracle on earth. That God instituted a living oracle in the Roman Magisterium.

Put another way, the target of sola scripture is a rival revelation and superior revelation. In Catholicism, the Magisterium is the functional equivalent of divine revelation. Indeed, progressive revelation, which effectively abrogates the authority of Scripture. 

Understood that way, including extrabiblical evidence to establish the canon is entirely consonant with sola scriptura. That's not an ad hoc exception to the principle. 

v) Another problem with the Catholic trope is the idea that the only alternative to exclusively internal evidence for the canon is exclusively external evidence for the canon. They think the case for the canon is completely dependent on tradition. In my experience, they never even consider the internal evidence for Scripture. And their neglect makes sense, given their Catholic outlook. After all, if the canon of Scripture is ultimate determined by ecclesiastical fiat, then evidence for the canon is superfluous–be that internal or external. 

vi) Part of the problem is a failure to distinguish between tradition and evidence. When Protestants cite "traditions" as evidence for the canon, that's not an argument from authority but an appeal to historical or testimonial evidence. They aren't citing Jewish or patristic sources because, say, Josephus or some church father is an authority-figure, but because they're a good source of information. Protestants aren't implicitly falling back on ecclesiastical authority when they include patristic testimony for the authorship of certain NT books. Authority involves the notion of the power or prerogative to obligate or compel asset or behavior. That's a different principle than evidence. 

vii) A final problem is that because Catholic apologists default to the Magisterium, they neglect to appreciate the extent of internal evidence for the canon. But it's striking to consider how interwoven the books of Scripture are. To take a few examples, the Pentateuch and Historical books are interwoven. A continuous, cross-referential history. The Prophets are interwoven with the Pentateuch and Historical books, given all the cross-references. The Synoptic Gospels are interwoven. Luke/Acts are interwoven. The Pauline Epistles are interwoven. John's Gospel and 1-3 John are interwoven. The Gospels and Epistles are interwoven with Acts, due to the same participants. So there's a huge amount of internal evidence for the canon.

I'm not claiming that internal evidence is sufficient to establish the canon. I don't think that's necessary. But the canon of Scripture is a unit in a way that Catholic apologists fail to notice.