The ad hominem family of flaws are irrelevant personal attacks that are used with the intent to discredit an argument. In real life, there are times when a personal attack can be considered valid—if someone has a history of robbing banks then it would weaken their argument as to why they want to work as a security agent for a bank—but when it pertains to the LSAT, if you can identify an argument as having an ad hominem flaw then you can consider it to be invalid.

Argumentum ad Populum

Argumentum ad populum is the belief that truth can be determined by, more or less, putting it to a vote. Democracy is a very nice thing, but it doesn’t determine the truth. Polls are good for telling you what people think, not whether those arguments are valid.

Here are some examples of this fallacy:

PrepTest 28 (June 1999 LSAT), S1, Q9 (p324)
PrepTest 28 (June 1999 LSAT), S1, Q19 (p327)
PrepTest 32 (October 2000 LSAT), S4, Q13 (p141)

Genetic Fallacy

This is a fallacy where you dismiss something based on where it came from.

Discoveries from Archimedes should be dismissed because he made them in a bathtub and not a proper university laboratory.

PrepTest 30 (December 1999 LSAT), S2, Q26 (p61)

Appeal to Authority

This is using the opinion or position of an authority figure, or institution of authority, as a trump card over an actual argument.

Ignore any scientific ideas from anyone who didn’t get a Ph.D.

PrepTest 20 (October 1996 LSAT), S4, Q20 (p75)
June 2007 LSAT, S2, Q17

The flip side of the Ad hominem argument is the appeal to authority.

No True Scotsman

This is a “purity test” where you rebut an argument by saying that the offender really wasn’t a member of your group.

The ideas from that member of the club are wrong, but he isn’t actually a true member of the club so his ideas do not matter.

Person 1: If you are a Marvel fan then you love Avengers: Endgame.
Person 2: I am a Marvel fan and I did not love Avengers: Endgame.
Person 1: Well you aren’t a TRUE Marvel fan.”

Next LSAT: September 21st

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Next LSAT: September 21st