In a discussion of atheism and ethics at Puritas, I noticed among the comments two very similar arguments about different subjects that commit the same fallacy. We had it drilled into us in William Alston's epistemology class, so I'm trained to notice it whenever it appears, but I notice a tone-deafness to this kind of distinction among people of certain types of views.
The fallacy consists of confusing metaphysics and epistemology. For non-philosophers, metaphysics is philosophy issues about reality, and epistemology is philosophical examination of knowledge. Here are some examples of arguments that confuse the two.
1. According to reliabilism in epistemology, you can know something (roughly) just by having a reliable belief-forming process that reliably leads to true beliefs.
2. But you can't know that the belief-forming process is reliable, because maybe it makes mistakes along the way, and you'd be in the dark about such mistakes.
3. Therefore, reliabilism must be false.
The metaphysical account of knowledge is statement 1. It explains what must be true for something to be knowledge. Statement 2 comes along and asks a further question about how you might know that it's knowledge. But that's a separate question. What makes it knowledge and how you know it's knowledge are separate questions. The first is metaphysics, and the second is epistemology.
Andrew's post offered an argument about how we might know moral truths. He argued against the likelihood that we would know about morality if atheism is true. Whatever else you might say about this argument, it's simply a change in subject to object by presenting problems with Divine Command Theory, which is a view about what makes moral truths true. Whatever problems Divine Command Theory faces and whatever problems Andrew's epistemological view might have, they aren't the same view. They aren't even about the same subject.
It struck me as noteworthy that the same confusion arose in the same conversation about a completely different issue. Andrew was pointing to divine revelation as one source of knowledge about morality, which led to some objections I often see against Protestant views of scripture. One complaint about Protestant views of scripture is that without tradition as an authoritative source you can't have an independent verification of scripture as infallible. On one level is the same sort of argument I discussed above. Someone claims scripture to be infallible. An objector comes along and acts as if our inability to prove that it's infallible undermines its infallibility. It can do no such thing. It may raise questions about how someone can claim to know of its infallibility, but not knowing its infallibility (and certainly not showing its infallibility) is irrelevant to whether it is infallible. The objection confuses our epistemic status about the revelation with a feature of the revelation itself.
But there's another level of the same problem going on here as well. Which scripture do we go with? The claim of Orthodox and Catholics is that tradition is necessary to establish that scripture can serve as an infallible source but also to establish which scripture is infallible. One difficulty with that view is that it just pushes the problem back. How do we know that tradition is infallible, if it is? If it's not, then it's at best just reliable, and then we can only rely on it for knowledge if reliabilism is true. But reliabilism needs no tradition. It just needs a reliable scripture. Why go the extra level? So calling for a proof that scripture is reliable commits the first error I pointed out.
At the same time, the metaphysical status of tradition may well be that it's reliable and produces true beliefs in a reliable way, and thus its determination of the canon may be knowledge. At least the one that gets it right would be. The tradition that doesn't wouldn't be. And we need not establish which is the right canon for this to be so. We just need to rely on the correct tradition and believe its reliably-produced statements (reliable because produced by God in his overarching providential plan). So it's interesting that several arguments about very different matters all commit the same mistake of confusing metaphysics and epistemology, and then when people combine those very different arguments into one you get something like these arguments against Protestant views of scripture that make the mistake in several different places.