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7 Logical Fallacies Lawyers Need to Stop Using

By Casey C. Sullivan, Esq. | Last updated on

Argumentation is part of any lawyer's DNA. It's essential to the profession and it's often something lawyers deeply enjoy. But good argumentation means avoiding -- or at least recognizing -- the hundreds of logical fallacies that can work their way into an exchange.

Of course, one man's sophistry is another's effective argument. But errors in logic can undermine the force of an otherwise strong position. Whether you want to avoid relying on fallacies, or simply want to call other attorneys out on their BS, here are seven logical fallacies attorneys use much too often:

1. Argument from Ignorance -- maintains that something is true because it has not been proved false or false because it hasn't been proved true. Think of McCarthy's declaration that "there is nothing in the files" of a State Department employee "to disprove his Communist connections."

2. Ad Hominem Attacks -- Ad hominem attacks attempt to undermine an argument by attacking the character of a person rather than the strength of his or her argument. When it comes to evidence, ad hominem attacks are barred by the general prohibition of character evidence -- and allowed by the many exceptions to that rule.

3. Argument from Force -- This fallacy seeks to gain compliance through threats of force, legal or otherwise. Threats as a persuasive tactic -- whoever would do such a thing!? Just anyone who has ever sent a demand letter.

4. Appeals Ad Annis -- Also known as "chronological snobbery," this looks to the age of something as proof or disproof of its truth. In fact, this fallacy is actually written into the Federal Rules of Evidence as the ancient documents rule, though its time many be running out.

5. Slippery Slope Argumentation -- This is the idea that small changes are a total surrender. It's a favorite of appellate briefs and judicial dissents. Justice Scalia has turned slippery slope arguments, and the subsequent "parade of horribles," into a veritable art form, as when he warned that decriminalizing sexually active gay relationships would usher in an age of prostitution and recreational heroin.

6. The Prosecutor's Fallacy -- Some fallacies are so ingrained in the profession that they're named after it! In the prosecutor's fallacy, the probability of a random match is presumed to be the same as the probability of guilt or innocence. So, imagine that a criminal's blood was found at a crime scene. It's type B positive. The Defendant has B positive blood. Only 10 percent of the population has that blood type. The prosecutor's fallacy posits that the defendant's probability of being guilty is then 90 percent.

7. The Defense Attorney's Fallacy -- The defense attorney's is the opposite of the prosecutor's. Under this fallacy, if 10 percent of the population has B positive blood, then there's no way that evidence implicates the defendant! After all, any of that tenth of the population could have done it.

Spot these fallacies in your opponents' arguments and you'll be set to take them down. Use them in your own reasoning and you'll certainly look like a fool. At least that's great logicians say -- and if they weren't right, well, they wouldn't be great, now would they?

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