There's an interesting question here:

"...demonstrate that your opponent is /trying to counter your argument/ by attacking..."

However, counterargument is not the only way to counter an argument. You can counter an argument by changing the subject, so you don't have to deal with it. So the question is, if one changes the subject from a substantive matter to personal attacks, are those attacks ad hominem? Because if one changes the subject to an unrelated substantive matter, that's a non sequitur.

Hmm, reminds me of a post I wrote two years earlier.

Incidentally, I think one of Bond's "real life examples" exposes an important ambiguity:

A: "I can even handle misplaced apostrophes every now and then. Not excessive amounts of them, [...]" B: "Perhaps double-check your grammar before you write a grammar rant that refers to 'amounts of apostrophes'." C: " ...the ad hominem nature of [B's reply] takes the sanctimonious angle that any who criticize must be without stain."

Bond writes, "B's reply was not ad hominem. It was not a counter-argument to A, but an attempt to point out what B saw as A's hypocrisy."

But actually it is ad hominem, i.e. directed 'to the person', though not (of course) an ad hominem fallacy in the usual sense. See: Ad hominem tu quoque.

I just finished a course in critical reasoning, this was one of the main fallacies that was addressed. I suppose what makes this different is that the insult actually directly applies to the situation. I would like to see some more fallacies covered in the future.

Is this an example of an "ad hominem" argument? Is it legitimate?

Bill: My amazing new medical breakthrough, explained in detail on my web page, will cure cancer! Joe: Bill is a known liar and con artist who has no special expertise in medicine. His so-called breakthrough is almost certainly worthless, and I'm not going to waste my time reading his page and writing a rebuttal.

To quote the page:

There is a false premise that lies behind all ad hominem arguments: the notion that all people of type X make bad arguments.

Probabilistically speaking, we only need to know that people of type X are more likely to make good-seeming bad arguments than average, to reduce our support of their arguments. We went over this once before.

I disagree with much of what is in the linked essay. One doesn't have to explicitly state an ad hominim premise to be arguing ad hominimly. Any non sequitur that is coincidentally designed to lower an arguer’s status is ad hominim in my book. Those statements have no other purpose but to create a silent premise: "My opponent is a tainted, therefore his arguments are bad." One can make ad hominim statements without actually saying them by using innuendo.

On the other hand ad hominim isn't even necessarily a fallacy. Of course an argument cannot become wrong just because a stupid person says it but we can expect that on average people with a bad track record in arguing will continue to argue poorly and people with good track records will argue well. In that sense we can set priors for someone's arguments being right before hearing them. Just remember to update afterwards. We actually do this all the time whether we admit it or not. We trust more what someone with a PHD in physics has to say about physics than a creationist. Saying that an argument is wrong because a stupid/bad person said it is of course fallacious, it's an attempt to reverse stupidity to get intelligence. However expecting people who normally say stupid things to continue to do so is Bayes compliant.

I see the ad hominim "fallacy" concept as more of an injunction or a hack if you will for human reasoners. It reminds us to examine the substance of the arguments of people we disagree with instead of dismissing them for political reasons. A perfect Bayesian mind could set up priors for people being right and impartially examine their arguments and update correctly without being swept up by political instincts. For humans on the other hand it might be more practical to focus on the substance exclusively and not the messengers unless the gap of expertise is huge (eg. PHD physicist vs. creationist on physics).

"Of course an argument cannot become wrong just because a stupid person says it but we can expect that on average people with a bad track record in arguing will continue to argue poorly and people with good track records will argue well. In that sense we can set priors for someone's arguments being right before hearing them." - PK

Does using someone's arguing track record violate the principle of charity, though? It's a matter of pragmatism to keep score in day to day life, it allows us to filter the noise and avoid information overload. However, if the goal is philosophical rigor and stringency, I think it's necessary to listen to each premise and argument.

Slight correction. I said: "Saying that an argument is wrong because a stupid/bad person said it is of course fallacious, it's an attempt to reverse stupidity to get intelligence." I worded this sentence badly. I mean that stupid people saying things cannot make something false and usually when people commit this fallacy it's because they are trying to say that the opposite of the "bad" point is true. This is why I said it's an attempt to reverse stupidity to get intelligence.

Basically when we see "a stupid person said this" being advanced as proof that something is false, we can expect a reverse stupidity to get intelligence fallicy right after.

Hm, I'm surprised nobody has mentioned the evidentiary reasoning version of ad hominem, something like "you seem to have a bad character, so I'm going to assign low weight to anything you say". I use this rule all the time. Ie, I'll give more weight to statements made in a reputable scientific journal than those on a Nazi website. This is not a valid argument againt anything on the Nazi website, just a rule that says not to pay too much attention to stuff found there, or at least seek independent verification from a more reputable source. There ought to be a fancy rhetorical term for this...there is for the opposite. Authority of sources is an important and under-described part of practical reason, and attacking the authority of a source is a pretty common form of argument that is quasi-ad-hominem but valid.

Off topic, but Eliezer, I think you have to read this paper immediately. I don't know how else to bring this to your attention, so I'm posting it here.

Layman's version: http://www.livescience.com/strangenews/081024-sb-theory-of-everything.html

I haven't read the actual paper, but it seems like something that could be very important.

Doug, AFAICT, this just says that you can't actually implement a Laplacian Oracle that by means of reliable abstract reasoning returns in general the correct answer to any predictive physical question, because you can just ask it "Predict whether I will raise my left hand or right hand" and then do the opposite.

I liked this made-up example:

A: "All rodents are mammals, but a weasel isn't a rodent, so it can't be a mammal." B: "Well, you're a rodent and a weasel, so there goes your argument."

B's reply is ad hominem, but it's also funny.

I didn't like this one:

A: "Listen up, asshole. All rodents are mammals, but a weasel isn't a rodent, so it can't be a mammal." B: "Yet another ad hominem argument. Ignore this one, folks." A is abusive, and his argument is fallacious, but it's not ad hominem. B's reply, ironically, is ad hominem...

Well, it's not that I dind't like the example itself. It's just that, given the lack of context, I don't see how B's reply is ad hominem. B's reply does not say "Yet another ad hominem argument by that notorious ad hominem arguer A", which would be ad hominem. It says "Yet another ad hominem argument", period. If this exchange took place in a discussion in which B was trying to portray A as a fallacious arguer, it would be ad hominem, but we can't reach this conclusion with the information we are given.

The author misuses the term ad hominem, so just reject all his arguments!

(By the way, B's argument is wrong because A's argument is not really ad hominem... but if it were, he would be right when he says it should be rejected. That's what we should do with fallacious arguments in general: reject them)

I think "ad hominem" reasoning makes a great deal more sense when you're dealing with someone citing alleged facts not readily verified. It makes a lot less sense when you're dealing with moral arguments, steps of reasoning from facts already known, etc. Then you want to just say - "If my opponent is flawed, let me prove it by finding the specific flaws in the argument."

No, it's not like this in theory, but the practice makes a good deal of sense. I had to give someone this advice just recently. It's extremely important to folks like us who've learned a large catalog of flaws that other people can potentially have.

Two separate issues:

1) Is it a good (legitimately persuasive) argument?

2) If not then after all the hairsplitting is done, what sort of bad argument is it?

The more important issue is (a). A few points:

a) Quibbling over the categorization of the fallacy is sometimes used to mask the fact that it's a bad argument.

b) There are plenty of people who can recognize bad arguments without knowing anything about the names of the fallacies, which leads to

c) We learn the names of the fallacies, not in order to learn to spot bad arguments, but as a convenience so that we don't have to explain at length to the other guy why the argument is bad.

d) Often perfectly legitimate arguments technically fall into one of the categories of fallacy. Technically being a classical fallacy is no guaranteed that an argument is actually fallacious. Some counterexamples.

In short, the classical fallacies are a convenient timesaver. But you don't need to have learned them to avoid being an idiot, and learning them will not stop you from being an idiot, and taking them too seriously can make you into an idiot.

While arguing on a message board where the accusation of ad hominem was undeserved I came up with a rather pithy example of what it actually is:

Einstein: E=MC2

Hitler: No it doesn't.

Einstein: Why not?

Hitler: BECAUSE YOU'RE A JEW!

(I'm now tempted to make a joke about Robin, Eliezer, Signalling, and a bottle of Jack Daniels - but I think I'll let good taste win out).

The link, at top, to "Stephen Bond's ... definitive word on ad hominem in 'the ad hominem fallacy fallacy' " is broken.  That excellent essay can be found at https://laurencetennant.com/bonds/adhominem.html

I suggest that the post, at top, be fixed.  Is it the LessWrong practice to fix broken links in 13 year old posts?