A fallacy is an error in reasoning that undermines an argument. It can be formal, meaning the structure of the argument is invalid, or informal, meaning the content or context makes the reasoning bad. Formal fallacies include affirming the consequent and denying the antecedent. Informal fallacies include ad hominem, straw man, false dilemma, and appeal to authority. Knowing fallacies helps you spot bad arguments and avoid making them yourself.

Fallacies are not always obvious. A straw man misrepresents an opponent's position. A false dilemma presents two options when more exist. An appeal to authority cites an expert outside their field. A slippery slope claims one step leads inevitably to disaster. Some fallacies are persuasive because they exploit cognitive biases. The ad hominem attack feels relevant even though it is not. The bandwagon appeal plays on the desire to belong. Critical thinking involves learning to recognize these patterns and asking whether the argument holds up without them.

Common fallacies

• Ad hominem. Attacking the person, not the argument.
• Straw man. Misrepresenting an opponent's position.
• False dilemma. Presenting two options when more exist.
• Appeal to authority. Citing an expert outside their field.
• Slippery slope. Claiming one step leads inevitably to disaster.

Not every bad argument contains a named fallacy. Sometimes the reasoning is just weak. Fallacies are a tool for diagnosis, not a substitute for judgment.