A logical fallacy, as the term is usually understood, is an inadequate argument presented in support of a position, usually in the implied context of a debate. The argument is inadequate precisely because it does not support the position, for whatever reason. Why such a failure is called "logical" is not entirely clear: presumably because the discipline of logic had its roots in such debates, in trying to codify the cases when a particular conclusion was valid or invalid given certain premises.

The history of logical fallacies is a long one, stretching back to the Greco-Roman civilization. We might ask why they have received so much attention over the years, with lists being carefully drawn up, promulgated and pored over. Nowadays, the history, poetry, and drama of ancient times are quite unpopular, yet the ancient fallacies are still eagerly discussed. In Ancient Greece and particularly Rome, rhetoric, the art of making convincing speeches, was valued highly and studied intensively. As part of one's training, one would of course learn to identify and avoid (or employ in the most effective way?) all the classic fallacies. But clearly the study of fallacies feeds human needs much more durable than the outdated art of formal rhetoric.

These needs are easily identified as the Path of Least Resistance -- the human preference for a quick and decisive, though shallow, victory, over a deeper but knottier consideration of the issues -- and, above all, the joyous, self-affirming experience of finding and exposing someone else's mistake. (This really needs a long German word to describe it, like Schadenfreude.) It's much easier to identify a logical fallacy than to seriously engage with ideas. And, for every fallacy spotted, one gets the pleasure of patting one's own back for being so much more intelligent and objective than the other fellow.

With these psychological incentives, no wonder that sniping at logical fallacies remains a popular tactic. But like other blood sports, it runs the risk of becoming the unspeakable in pursuit of the undefensible. A logical fallacy, ineffective though it may be, is at least an attempt to contribute to one side of a debate. Pointing out your opponent's logical fallacies in detail is one more step away from saying something relevant to the discussion. Although I have not experienced it first-hand, I am reliably informed that USEnet provides good examples of this descent from meaningful debate into quibbling. (Of course, if you like quibbling, that's different.)

So, what to do if your opponent does happen to use a logical fallacy? The only honourable course of action is to resist the temptation to exult or crow, but reply with a valid counter-argument which exposes by its own strength the weakness of the fallacy.

Not this:
A -- Plato says that goat's milk yoghurt is better than cow's milk yoghurt. But Plato is a knock-kneed, grimy old fool, who has a reason to prefer goats which I can't explain in front of the children, if you catch my drift. Cow's milk yoghurt for ever!
B -- Aha, aha, that was an obvious argumentum ad hominem combined with a vile innuendo. Your fallacies are exposed for all to see. You lose, sucker!

But this:
A -- (Same thing)
B -- Plato may be all of those things, but how can that alter the fact that goat's milk yoghurt is creamier, less acidic and contains a larger concentration of cancer-preventing antioxidants?1

Above all, do not commit the fallacy of imagining or claiming that you are right and your opponent is wrong because he or she has used a fallacy. Your ability to spot fallacies makes you a clever bastard, but does not make you right.

Being a clever bastard, and in the spirit of Godel's Theorem, I would like to call this argumentum ad fallaciem, the attempt to substitute fallacy-spotting for substantive argument. (This is clearly a subdivision of the fallacy of Style over Substance, in which one tries to argue that one's opponent is wrong because of the way in which he or she is arguing.) Any time an argument gets clogged up with "straw men", just mutter "argumentum ad fallaciem" and move on quickly.

1. I do not know whether either type of yoghurt contains cancer-preventing antioxidants. However, goat's milk yoghurt is delicious, and anyone who disagrees with that can go to Hades.