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Today at a Glance:

Logical fallacies are errors in reasoning that undermine the quality of an argument.

Combatting them relies first and foremost on establishing a level of awareness—both academically and practically.

The Logical Fallacy Guide covers 20 common logical fallacies: Ad Hominem, Texas Sharpshooter, Sunk Cost Fallacy, Bandwagon Fallacy, Straw Man, Appeal to Authority, Post Hoc Ergo Propter Hoc, Personal Incredulity, False Dilemma, Burden of Proof, Red Herring, No True Scotsman, Hasty Generalization, Non-Sequitur, Tu Quoque, Slippery Slope, Begging the Question, Loaded Question, Equivocation, and Fallacy Fallacy.

The Logical Fallacy Guide

If you’ve been reading this newsletter, you know that I like to say that humans are fascinating creatures.

We possess the capacity to accomplish some complex feat of technology and engineering, and subsequently fall victim to the most obviously flawed base logic.

Logical fallacies—errors in reasoning that undermine the quality of an argument—are classic examples of this fact.

The Merriam-Webster Dictionary defines fallacy as a false or misleading idea. A logical fallacy, therefore, can simply be thought of as logic based on a false or misleading idea.

Unfortunately, unless you went to law school—or took a robust philosophy course load in college—you’ve likely been minimally exposed to them in a formal context.

Accordingly, we frequently fall victim to logical fallacies—our own emotional, psychological, and intellectual blindspots create the cracks and we fall right into them.

There is no such thing as a perfect logician, but we can all strive to cover our blindspots and craft better arguments. Similar to the study of cognitive biases—which I’ve written about recently here and here—the first step in avoiding logical fallacies is developing an awareness of them.

In that vein, today’s piece will cover 20 common logical fallacies to learn, identify, and avoid.

Without further ado, let’s dive in…

Ad Hominem

Latin phrase for "to the person”—an ad hominem attack is an attack of the individual rather than the argument.

Instead of addressing the argument—its structure, logic, and merits—the offender attempts to refute the opposition on the basis of personal characteristics.

It may be overt—openly attacking the person’s character or personality—or covert—subtly doing the same—but it always focuses on the person, not the argument.

Often referred to as “mud-slinging” in political circles, if you’ve ever watched a political debate or political campaign ads, you’re already familiar with this one. It’s all-too-common on Twitter and other online discourse, as well.

Example

Candidate 1: “…and this is why I believe we need to implement a much more aggressive set of climate change regulations.”

Candidate 2: “I’m sorry, but are we really expected to believe anything coming from a known liar who cheated on his college entrance exams to get to this position?”

The offender (Candidate 2) has attacked Candidate 1 the individual, rather than the argument itself.

The Texas Sharpshooter

The name of this fallacy is based on a fable:

A Texan fires a gun multiple times at a barn wall. He then walks over to the bullet-riddled wall and paints a target around the closest cluster of bullet holes to create the appearance of impressive marksmanship.

Think of this as cherry-picking—selecting and highlighting evidence that supports the conclusion and systematically ignoring evidence that may refute it.

Example

“Tara is a really impressive and successful restauranteur. Her restaurant on Park Avenue is always full and gets really high ratings on Yelp.”

This may be true, but it ignores the fact that Tara’s five other restaurant openings have failed. The cherry-picked data—the successful Park Avenue restaurant—is used to draw a broad conclusion about Tara’s quality as a restauranteur that may be inaccurate.

Sunk Cost Fallacy

A favorite of behavioral economics.

Sunk costs are the economic costs already invested in an activity that cannot be recovered. Money spent on non-refundable flights and hotels, time invested in a project, or energy put towards a relationship all qualify as sunk costs.

The fallacy is found in thinking that you should continue with something on the basis of all that you've put in, with no regard given for future costs or likelihood of ultimate success.

The reality: sunk costs are irrecoverable, so should not factor into the decision about the future.

Example

“The hopes for the space project appear slim, but we have already invested so much, so we have to finish.”

If the space project appears unlikely to achieve its stated objectives, any additional investment in the project may be irrational. The fact that a lot has been invested in it has no bearing on what should rationally be invested in its future.

“I really don’t want to go on this vacation—I’m so busy at work—but I already paid for the flights, so I might as well go.”

The cost of the flights is a sunk cost. If going on the vacation is going to bring you negative utility, that should be the only factor in your decision of whether or not to make the trip.

Bandwagon Fallacy

An assumption of truth on the basis of the majority of people believing it to be true.

"Everyone believes X, so obviously X is true."

The assumption is typically made without regard for the qualifications or ability of the people in question to validate the claim.

Common in the workplace, where a collective belief—“this is how everyone does it”—can lead to broken processes and systems.

Example

"Well, the majority of people we talk to say that integrating chip design and manufacturing is important, so clearly you should get onboard so we can continue forward with our integrated solution, rather than switching to something modular.”

The over-reliance on the majority—with no regard for whether these people are qualified to make this decision—may lead to poor judgement. This is where first principles thinking goes to die.

Straw Man

Setup a straw man to tear down.

The offender ignores the...