I would think, in general, that an ad hominem doesn’t show that a particular claim is false or even that it is probably false; it merely undermines the evidence that has been presented for that claim. As I said, that doesn’t show the claim is false: a bad argument can have a true conclusion.
S&I, in my understanding, an ad hominem cannot possibly undermine evidence. Evidence is facts, observations, physical findings. They stand or fall on their own.
w.
An Ad Hominem argument can (but of course does not always) successfully undermine evidence. If my basis for believing something is at least in part “That X told me so,” (and this can often be a perfectly good basis for belief,) then if X can be relevantly impeached as a reliable relayer of testimony in this case, then at least part of the basis for my belief has been undermined. Assuming any reasonable basis for belief counts as evidence, then it follows that at least part of my evidence has been undermined.
Observations don’t stand on their own–they are based on presumptions, for example, that the instruments used to make them are calibrated correctly. (If you want to count as “observations” only the actual act of reading those instruments, then there is still a presumption that I’ve actually read them correctly–that nothing has distorted my eyesight, that I haven’t had a momentary brain-fart, and so on.) So even if I don’t have a direct counter-observation showing the original one to be false, I can still show the original one to be unreliable, if i can show that there was something substandard about the observation process or the one undergoing it at the time.
Facts stand on their own in a sense, but claims to fact do not. There are always presuppositions behind any claim of fact, and successfully bringing those presuppositions into doubt can sucessfully undermine–show unreliable–the claim of fact. (I.e., even without there existing some claim of fact directly negating the original one.)
I’m not sure what the distinction between a “physical finding” and an “observation” would be, but if anything, a “physical finding” is even more undermineable than an observation, since I would think a conclusion based on reasoning from data counts as a “physical finding” while I think most people (I am not one of these people) think an observation can not involve reasoning from data. It is clear that a bit of reasoning based on data can be undermined–shown unreliable–even in the absence of some other physical finding directly negating the first one.
Finally, I would have counted testimony as a form of evidence. Why don’t you?
-FrL-
By the way, I think that people are talking past each other in this thread. I think that what counts as evidence differs in scientific, legal, and everyday contexts, and I think there are individuals with each of these contexts at least implicitly in mind as they post.
The context I’ve had in mind has been a muddled mishmash of the three.
-FrL-