One of the most common missteps in persuasive speaking is using logical fallacies. Simply put, these are arguments lacking in sound logic. Plenty of arguments that are fallacious or otherwise flawed are in fact widely accepted, say debate experts Kate Shuster and John Meany in their book On That Point: An Introduction to Parliamentary Debate. For example, the fallacy known as slippery slope, described below, appears repeatedly in public policy speeches.
Here are a few common logical fallacies to avoid in your next speech—or to be aware of when someone tries to persuade you of something.
- Slippery slope. This argument contends that events will set off an uncontrollable chain reaction when there is no real reason to expect that reaction will occur. Example: “If we start regulating carbon dioxide, the next thing you know the Proposition Team will be telling you what to eat for breakfast.”
- False dichotomy. This fallacy occurs when an argument presents two alternatives and suggests that it is impossible to do both, or that they are mutually exclusive—e.g., “It’s either free school lunches or nuclear war”; “Either you let me go to the concert or my life will be ruined.”
- Appeal to ignorance. When an argument has not been disproved, it does not therefore follow that it is true. Yet the appeal to ignorance works a surprisingly large amount of the time, say Shuster and Meany, particularly in conspiracy theories. “No one has yet proven that aliens have not landed on Earth, therefore our theory about ongoing colonization should be taken seriously.”
- Simplistic appeal to emotions. Speakers routinely try to play on the emotions of the crowd by saying something simplistic and melodramatic, instead of telling engaging stories or making real arguments. “I know this national missile defense plan has its detractors, but won’t someone please think of the children?”
- Red herring. An old standby, the red herring is an attempt by the arguer to divert attention to another issue and then draw a conclusion based on that diversion. “The candidate has a weak stand on education: Just look at what she says about foreign policy.”
Dave Zielinski is a freelance writer in Minneapolis, Minnesota, and a frequent contributor to the Toastmaster magazine.