Many readers of this site know that the fallacious variety of ad hominem argument admits of many different types–the abusive, the circumstantial, the tu quoque, and much much more. Some friends of The Non Sequitur, Scott Aikin and Bob Talisse, have written an illuminating and entertaining piece on the tu quoque variety for the magazine Scientific American Mind. Read it here.
For those who don't remember, one is guilty of the the ad hominem tu quoque variety of fallacy when one charges one's opponent with hyprocrisy when such hypocrisy is irrelevant to the strength, cogency, validity or whatever of the opponent's argument. Al Gore's riding in a private jet does not make Al Gore's claims about global warming false. Sure, it's rhetorically effective to talk about Al Gore's private jet riding, it's even fun, but so are a lot of sophistries. That's why they're called "sophistries." But, like all informal fallacies, it's a kind of cheating–and cheating is a kind of stealing in that one claims to have demonstrated what one clearly has not–and, as the mindless dogma of liberal academia has it, stealing is wrong. In this case, one claims to have shown something about climate change by showing something about Al Gore. To say anything worthwhile about climate change, however, you have to do the necessary work–pointing out Al Gore's electricity usage does not count.
In addition to discussing the general questions of relevance central to the definition of the ad hominem tu quoque argument, Aikin and Talisse also point out that sometimes the hyprocisy may underscore rather than contradict the hypocrite's point. So, for instance, if a smoker recommends someone quit or not start for health reasons, we might look at that apparent hypocrisy more creatively. Their not being able to quit underscores one of the dangers of smoking.
In addition to that insightful point, what is most interesting about the article are the clueless and unhinged responses in the comment section. Some people simply cannot read the four letter string G-O-R-E without losing it.