Most epistemologists recognize that many of the common informal fallacies are epistemically relevant in various ways. Thus, even though tu quoque arguments are irrelevant to the truth of the position being espoused, they are relevant epistemically because one way to provide supporting evidence for a view is to be able to explain away why intelligent people might disagree.
Recently, there has been a flurry of activity in blogdom about the genetic fallacy and whether Dan Dennett committed it in his latest work on the origins of religious belief. Some say he did, some say he didn’t. Those who say he didn’t point out that finding unsavory origins for a belief is epistemically relevant even if it is not relevant to the truth or falsity of a belief.
Well, so it is, sometimes at least. But, of course, any good epistemologist will want a fully general account here, and it is certainly false that finding unsavory origins for a belief always undermines any warrant or justification one might have for the belief.
In short, unsavory origins do not imply irrationality or lack of justification or warrant. My father thinks there were more earthquakes in the 20th century than in any previous century because it fits his wacky eschatology. I grew up thinking so as well, and still do. I inherited the opinion by testimony from the unsavory origins just described. But I also know it to be true: there’s a quite neat plate tectonics explanation for it. There’s nothing at all epistemically untoward that afflicts my belief because of its unsavory origins.
Or think of Foley’s Swampman, who arises fully formed both physically and doxastically out of a lightning strike in the swamp. Ask him anything and he will give you a correct answer. His grasp of truth far exceeds any of ours, and Foley concludes from this that Swampman knows a lot more than you or I. I’m not sure this is correct, but what I do think is correct is that he is epistemically superior in some really important way. I’d prefer to say his understanding far exceeds our own. But whatever we say, we shouldn’t attribute irrationality or lack of justification to the guy because of our knowledge of the origins of his beliefs.
Even so, there are lots of cases where origin makes a belief epistemically suspect. So the question is, for we fans of full generality, what’s the difference between cases where origins cast aspersions on a belief and cases where they don’t?