Learn to Identify the Ad Hominem Fallacy, You Jerk

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Everyone knows you should never take health instructions from an overweight doctor. Don’t go see the new Tom Cruise film — he’s a Scientologist. And never, but never, take lawn-upkeep advice from one of them Libertarians.

Each of these arguments rely on the same logical fallacy: Ad Hominem. They’re common, and if you’ve ever watched Glenn Beck you probably appreciate the need for their dismissal.

We have something of a tendency to allow these fallacies to corrupt our thinking in standard discourse — we may look less favorably on New York’s financial regulation after learning of Elliot Spitzer’s prostitution scandal, for example. Ultimately, however, arguments on the LSAT should be judged on their merit, not on the alleged character of the arguer. So if you have a question regarding the physics of suspension bridges, and learn that the project’s engineer wears only the furs of cute and endangered animals, you probably shouldn’t factor that horrific detail into your assessment of the bridge construction.

It’s important to recognize the nuance behind this particular fallacy. In some instances, consideration of an individual’s character may be relevant to the discussion, as below:

Dan claims that timeshares in Boise, Idaho would be a prudent financial investment, but Dan is deceitful and has lured many people into unprofitable schemes before, so I should not invest.

Here we have a relevant concern in the context of the argument. Dan’s past behavior factors into our decision calculus, and it should. If one of the answers on a Weaken question elucidated these points about Dan’s habits, it could effectively weaken the conclusion that you should invest in his company. Similarly, if the previous example was about regulating the fur trade, then our pitiless-panda-purchasing-person might have character baggage worth considering.

By contrast, if Dan is just recommending a grass-mowing technique, then his shady business practices are less of a concern and we may want to take him up on his suggestion. That is, if he’s not a Libertarian, of course.

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