Slippery slope? Post hoc, ergo propter hoc? A false dilemma, perhaps? These and scores of other terms designate logical fallacies, types of erroneous reasoning that unseat an argument. We've all heard them and undoubtedly used some. Yet they have no place in a sound argument, a type of discourse in which reasoning persuades an audience to support a view or to take an action.

"Fallacy" comes from the Latin for "trick" or "deception." That’s what logical fallacies do: They trick the heedless into thinking they've heard a rock-solid argument, not one that contains errors in reasoning.

While we may be most accustomed to recognizing logical fallacies in advertising or politics, they appear in the sciences and humanities, in business, in journalism, everywhere. Fallacies will dispose a mindful audience to reject your argument, so scour your presentations for them. Qualify or revise your claims as necessary to stay within the realm of sound logic.


The slippery slope fallacy suggests that doing one thing leads to another and then another, a chain reaction that eventually lands us somewhere we do not want to be. Thus, the fallacy claims, we'd better not do that first thing. But it provides no evidence that such a chain reaction will occur.

Wanting to block a particular approach to developing a product, for example, a team member might invoke the slippery slope. "If we don’t do the focus group with seniors," he argues, "we won't be able to develop a targeted marketing campaign, consequently lowering our eventual market share, leading to the product’s failure."

He’s on the slippery slope unless he can provide researched support for the causal relationship between steps.


Although the post hoc, ergo propter hoc (after this, therefore because of this) fallacy insists otherwise, a causal connection does not necessarily exist between sequential events. Committing the post hoc fallacy, an arguer may assert that adding to the sales force will increase revenue. But will it? Perhaps improving communication between the sales force and customers or enhancing delivery efficiency or offering better incentives will then boost revenue. To establish causality between events, this arguer must provide evidence of causation, not simply assert that B will result from A because B will happen after A.


Presenting only two solutions to a problem when others exist is an either/or fallacy, also known as a false dilemma. In this fallacy, the two choices presented are mutually exclusive; often one is desirable while the other is not. "If you can’t represent our company in the webinar," a manager says to an employee who has developed a conflict, "we'll just cancel." Are those the only two options? Perhaps someone else can represent the company in the webinar. Perhaps the organizers would explore a scheduling change. Perhaps the representative can find a way to resolve the conflict.

The either/or fallacy often results from black and white thinking; to avoid the fallacy, try to think wider, more creatively.

An argument demands critical attention from both speakers and listeners. Both must be alert to errors in logic, to how they weaken persuasion, to how they reduce potency and thus belief. As a speaker, don't commit fallacies. As a listener, don't succumb to them.