Reductio good, slippery slope badKevin Drum has a post stating why he thinks reductio ad absurdum arguments are bad ones. He says rather than saying Principle X, carried to the extreme, yields an absurd result, everyone should acknowledge that competing principles and interests will yield different results that prevent reaching an absurd result.
Kevin's wrong in this case - he should be going after slippery slope arguments instead. Reductio is a legitimate argument to raise if someone else says Priniciple X is all one needs to decide Issue A. Responding with a reductio argument forces the person arguing for Principle X to say why the result is not absurd, or to acknowledge that Principle Y sometimes controls, and explain why Principle Y should prevent the absurd result but should not control Issue A.
Slippery slope arguments are different - they say that if you apply Principle X to Issue A, then no matter what, awful unintended consequences will follow. Slippery slope arguments just assume the exaggerated consequences will follow, without providing a reason for why they will happen. Reductio arguments are different - they don't assume awful things will happen, rather they argue that absurd/awful things would be justified on the basis of the principle their opponent supports. Reductio therefore has a logic to it, while slippery slope is just an argument people make up when they can't think of a persuasive reason to support their viewpoint about about the particular issue being discussed.
So much for my rant. Many others rant in the comment section to Kevin's post.