Consider the following argument form:
- p is believed because of argument A.
- If A is a good argument, then q.
- Not q.
- Therefore, not p.
- The Immaculate Conception (the doctrine that Mary was saved by grace alone in the first moment of her existence) is believed because of the argument that Jesus would be subject to original sin if Mary was.
- But if Mary's original sin would have infected Jesus, then the original sin of Mary's parents would have infected Mary, and so on, and hence all of Mary's ancestors would need to be without original sin.
- But not all of Mary's ancestors are without original sin.
- Therefore, the Immaculate Conception is false.
Notice, however, that there is an argument form in the vicinity of (1)-(4) that is better. Namely, we replace (4) with:
- Therefore, the belief that p is not justified.
However, if we do that, we need to read (1) as saying "p is only believed because of argument A". But people are not very good at figuring out the reasons why they themselves believe something, and are much worse at figuring out why others believe something (that explains why so much political discussion is heated—for it is often difficult for people to figure out why someone might take a political position that disagrees with theirs, unless that person is stupid or vicious). So if the implied epistemic agent in (1) and (9) is someone other than the person offering the argument (1)-(3),(9), the argument is likely to be unsound in step (1). I think that when the argument form is found in the wild, it is common for (1) to fail.
Furthermore, it is important to keep constant the epistemic agent constant between (1) and (9).
I think the Marian case likely screws up on all of these counts. First of all, in (5) the epistemic agent is likely some ordinary believer or at best an incautious rookie apologist. In the analogue of (9), the epistemic agent is the Church. Second, even in the case of the ordinary believer or the rookie apologist, (5) ceases to be true if we read "is believed" as "is only believed", because in fact the ordinary believer and the apologist believe in the Immaculate Conception not just on the basis of their flawed argument, but primarily on the basis of the Church's authority.
Moreover, arguments of the form (1)-(3),(9) are often annoying to the interlocutors. I remember a job interview where a candidate tried to convince me that I only rejected his or her view because of thinking about a topic under the influence of a certain other position. This bit of mind-reading not only was incorrect (I can be accused of many things, but not of that particular influence; I rejected the candidate's view cause the conjunction of its claims obviously entailed some absurd proposition that we were both committed to the denial of), but I found it annoying and a bit offensive, and the other members of the interview committee did not like it either. That was a somewhat different version of the genetic fallacy, but the genetic fallacy is generally annoying.
There are places for arguments of the form (1)-(3),(9). But when one gives such arguments, danger is all about.