ARGUMENT BY ABSURDITY: This comment thread from Mean Mr. Mustard encouraged me to write a little post on one of my favorite hobbies: argument by absurdity. Most people who hold opinions don't really understand what they believe (this goes for pretty much any belief system), and even if there are strong arguments in favor of their beliefs they generally aren't aware of them. Most often, they will state poorly formed versions of good arguments that miss some crucial points -- this is when you pounce.
There are two possible beginnings: sincerity or sarcasm. Sincerity can be fun if you're in a really wicked mood, because you can lead your mark along slowly and get them to agree with the conclusions you initially draw from their arguments. Sarcasm works too, but you have to strike more quickly because people tend to tune out sarcastic responses before you can get to the good stuff. Once you've found the flaw in their reasoning (whether you agree with their position or not is beside the point) decide how you will exploit it, and either go for the quick kill with sacrasm or lead them along the garden path of sincerity before rhetorically stabbing them in the back.
You've decided on an initial approach, now what? The sky's the limit! What you really want to work towards is a totally absurd conclusion that logically flows from the arguments that they've made; you have to do so without revealing the true strength of their arguments however, assuming there is any. Do your best to prop up the strawmen they've constructed and to nourish their false dichotomies. Work your way carefully towards your absurdity and then just toss it out, doing your best to imply that your mark really believes it. If you're trying to be sincere, you can pause to consider the absurdity before rejecting it, thereby gaining some credibility just for appearing thoughtful.
In the end, once the absurdity is rejected by everyone involved make sure that you tie that rejection together with all your mark's arguments, so that they will be rejected as well. If you're clever, you can discredit all his reasoning, even the parts that are legitimate and well-founded. Any attempt he makes at defending himself can be easily brushed aside by pointing at the absurd conclusion, and most people won't give him another thought.