Logical Fallacies: A Basic Guide
Common reasoning errors, with plain-language definitions and examples.
A fallacy is an error in reasoning that makes an argument weaker or invalid. Recognising fallacies helps you write better arguments and evaluate the claims of others.
A conclusion can be true even when the argument for it is fallacious. A fallacy shows the argument is weak, not that the conclusion is false.
Common fallacies
Ad hominem (attacking the person). Rejecting an argument by attacking the speaker instead of the claim. "You can't trust her view on tax; she's rich."
Straw man. Misrepresenting someone's argument to make it easier to attack. A says: "We should spend more on public transport." B replies: "So you want to ban all cars."
False dilemma. Presenting only two options when more exist. "Either we cut all spending or the country goes bankrupt."
Slippery slope. Claiming one step will inevitably lead to extreme consequences without evidence. "If we allow a short extension, soon no deadline will mean anything."
Appeal to authority. Treating a claim as true only because an authority said so, especially when the authority lacks relevant expertise. Citing real experts on their subject is not a fallacy.
Appeal to popularity. Arguing that something is true because many people believe it.
Post hoc (false cause). Assuming that because B followed A, A caused B.
Circular reasoning. The conclusion is assumed in the premise. "The book is true because it says it is true."
Hasty generalisation. Drawing a broad conclusion from too few examples.
How to respond
- Identify the claim and the reasons given.
- Ask whether the reasons actually support the claim.
- Name the weakness politely and explain why.
Quick recap
A fallacy is a flaw in reasoning. Learn the common types, check whether reasons support conclusions, and remember a flawed argument does not make a conclusion false.