Definition of logical fallacy

abstract pattern of brain cells

A (logical) fallacy is basically ‘faulty logic’. Sometimes we are wrong because we have the wrong information. Perhaps the government has shut off your Internet, preventing access to the truth. Fallacy is a specific kind of mistaken belief: You arrive at a conclusion through faulty thinking.

Typically, logical fallacies sound okay until you look more closely.

Fallacious is the adjective form of fallacy.


Since fallacies always involve logic, you don’t really need to say ‘logical fallacy’. It’s redundant, like saying PIN number when the ‘n’ in PIN stands for number. However, you may still choose to use the phrase ‘logical fallacy’ because it’s in common use.

That’s what I’ll do here. The problem is, when someone says ‘logical fallacy’, they might be talking about all kinds of fallacy or they might be talking about only ‘formal fallacies’. (More on the difference below.)


Sometimes people use faulty logic to trick you. Sometimes they simply don’t know what they’re talking about.


Locate lists of fallacies at various places on the web. Here’s a good one at The Internet Encyclopedia of Philosophy. They have collected 231 ways of being wrong.

But they haven’t collected all of them! There’s no end to our human capacity for being wrong. New fallacies keep being named. For instance, in 2021, people suddenly started talking about a new “Motte & Bailey” fallacy.

(The fallacy was named earlier this century, but it took the Internet a while to pick it up. It’s named after a type of English castle.)


Informal fallacies concern inductive logic. Formal fallacies concern deductive logic.

  • Inductive fallacy: The logic doesn’t work because the person making an argument is making use of an action/belief/feeling (a warrant) to support an inference between two things (evidence and claim).
  • Deductive fallacy: In this case, the logic doesn’t work because the conclusion doesn’t (necessarily) follow from the premise.

Put another way:

Informal fallacies involve faulty inferences and/or false premises. They don’t concern the structure of argument itself. They’re very common in everyday life. Informal fallacies come about because people make use of irrelevant information. They assume. They misuse language and evidence.

Formal fallacies concern the form of the argument. (These fallacies violate the rules of Conditional and Categorical Syllogisms.)


  • Someone doesn’t buy that the climate is changing because of human activity. Their fallacious reasoning? Because climate scientists can’t even agree which year, exactly, the Earth will be plunged into climate disaster. If the scientists can’t even agree on that, who’s to say they’re right about any of it? This is an example of Inflation of Conflict. This inductive logic doesn’t work because the person arguing against anthropogenic climate change is making use of minor disagreements between scientists as evidence to support their own claim: That unless scientists agree 100% of the time about everything climate related, climate change isn’t happening.
  • A 25-year-old absolutely hates their job. But it took five years to qualify for it. Because they worked so hard to get it, they better do it for the next 40 years until retirement. This is an example of Sunk Cost Fallacy. This is an inductive fallacy because the 25-year-old feels five years of their life will have been wasted unless they make direct use of their training (the claim). The evidence? No evidence, just fear. The next phase of life may be equally difficult, expensive and unpleasant.
  • A transgender kid has a grandpa who won’t call them by their non-binary pronouns. Grandpa has never experienced gender dissonance in his life. In his day, men were men, women were women and nothing in between. Because he can’t imagine a non-binary gender, there’s no such thing. This is an example of Mind Projection. Grandpa thinks however he sees the world is how it really is. This is an example of an inductive fallacy because Grandpa’s beliefs about gender don’t support the inference between the claim (I don’t see, experience or understand gender dissonance) and the evidence (therefore it’s not a thing).

If your mother ever told you, “Two wrongs don’t make a right” she was cautioning you against the informal Fallacy of Relevance. (It’s tempting to think a wrong can be cancelled out by another wrong.)

If anyone’s ever told you, “That’s the pot calling the kettle black,” they’re leaning on the informal fallacy of Tu Quoque (literally: You too). Another name: Appeal to Hypocrisy. If you chuck rubbish out the car window, then tell your brother not to do it, your brother will ignore your hypocrisy. That doesn’t mean it’s okay to litter. It only means you’re both wrong.


  • All this week, if I hang the washing outside to dry, it rained. Therefore, if I hang the washing outside today, it will rain. This fallacy involves no induction — no inference. Just plain old faulty logic. So it is an example of a formal fallacy. Affirming the Consequent.
  • I paid $15k to buy my kid braces for their teeth. There’s another kid down the road who needs braces but his parents haven’t got him any. Guess they don’t care about their kid then, eh. This is a formal fallacy called Denying The Antecedent. I have denied the antecedent that the parents can’t afford braces because they are poor, assuming the lack of expensive dental intervention means they don’t care. My error of thinking goes like this: Because a implies b, then not a implies not b. (Because providing care for your kids implies caring, then not providing care for your kids implies not caring.)
  • Here’s another transphobic fallacy, a formal one this time based on messed-up logic: All boys have penises. My non-binary grandchild has a penis. Therefore he [sic] is a boy. This is an example of Undistributed Middle. The part left out of their logic: Everyone who has a penis is a boy. (This left-out part is the part that is not true, because transgender people exist.)


Sometimes we make use of formal fallacies and still come up with a correct conclusion. But that’s not because the logic works.

If we assume something must be untrue because the argument contains a logical fallacy, this in itself is a logical fallacy! This irony is known as The Fallacy Fallacy.

People who know a few things about fallacies tend to fall into this trap. “What you’re saying is a fallacy! Therefore you’re wrong!”

Nope. The person could — by coincidence? — still be completely right!


A friend tells you not to eat bread because it contains gluten and gluten is unhealthy for everybody. Let’s say the friend is wrong about the gluten, that celiacs are much better off avoiding gluten, but everyone else is unaffected by this family of proteins. You tell the friend that not everyone is gluten intolerant, therefore it’s fine for you to eat bread.

However! It may be the case that you’d still be better off cutting out bread for reasons unrelated to gluten. Perhaps your diet is carbohydrate heavy which means you don’t eat enough vegetables. Perhaps the bread you’re eating contains arsenic.

It would be an error in thinking to conclude that you must keep eating bread simply because your friend is wrong about the gluten.


Pathetic Fallacy: Not actually an insult.

When authors deliberately evoke pathos by linking emotion to an external event, this is not a logical fallacy as described above. In fiction, pathetic fallacy is a common and effective literary technique.

However, it would be an error of logic to assume that everyone in real life must be sad because it is raining, or happy because it is sunny out.