Logical Fallacies
A logical fallacy is a flaw in the structure or content of an argument that makes it invalid or misleading, even when it sounds convincing on the surface. Fallacies are not simply wrong facts — they are wrong moves in reasoning. Knowing the names and patterns of common fallacies is one of the most practical thinking skills you can develop. Once you recognize a fallacy, the whole argument loses its grip on you.
Why Fallacies Work
Fallacies succeed because they exploit mental shortcuts and emotional responses. When you are in a hurry, emotionally invested, or impressed by a confident speaker, your brain is more likely to skip the careful evaluation that would expose the flaw. Advertisers, politicians, and even well-meaning friends use fallacies constantly — sometimes deliberately, sometimes without realizing it. The goal of studying fallacies is not to win arguments by shouting 'fallacy!' at people, but to protect your own thinking from manipulation.
Six Essential Fallacies
Ad hominem means 'against the person' in Latin. This fallacy attacks the person making the argument rather than the argument itself. 'You cannot trust anything she says about nutrition — she used to eat junk food all the time.' Even if this fact about the person is true, it says nothing about whether her current argument is correct. The argument stands or falls on its own merits, not on the arguer's history. Straw man occurs when someone distorts or exaggerates the opposing position to make it easier to attack. 'You said we should reduce military spending — so you want to leave the country completely defenseless?' Reducing spending is not the same as eliminating defense. The straw man is a misrepresentation, not the actual claim. False dilemma (also called false dichotomy) presents only two options when in reality more options exist. 'Either you support this policy completely, or you do not care about the environment.' In reality, a person might support environmental protection through different policies entirely. Reducing a complex issue to two extremes forces a false choice. Appeal to authority uses the opinion of an authority figure as a substitute for real evidence. 'A famous actor says this supplement cures disease X, so it must work.' Fame in one domain does not transfer to expertise in another. The relevant question is whether the authority has genuine expertise in the specific topic. Slippery slope claims that one small step will inevitably lead to a catastrophic chain of events, without showing the links between steps. 'If we allow students to retake one test, soon nobody will study for anything and academic standards will collapse entirely.' Each step in the chain needs its own evidence. Post hoc (short for post hoc ergo propter hoc — 'after this, therefore because of this') assumes that because one event followed another, the first caused the second. 'I wore my lucky socks and we won the game. My socks caused the win.' Correlation in time does not establish causation.
Knowing the Latin and Greek names for fallacies (ad hominem, post hoc, non sequitur) gives you a shared vocabulary with anyone trained in critical thinking. When you say 'that is an ad hominem attack,' a knowledgeable listener immediately understands the precise flaw you have identified. These names are not trivia — they are precision tools.
Match each fallacy to its precise description.
Terms
Definitions
Drag terms onto their definitions, or click a term then click a definition to match.
Spotting Fallacies in Real Life
Fallacies appear everywhere once you know what to look for. Comment sections online are a rich source of ad hominem attacks. Political speeches often contain false dilemmas. Advertisements frequently commit appeal to authority by featuring celebrities rather than experts. News headlines sometimes imply post hoc causation that the article itself does not actually claim. The useful habit is to pause when an argument feels compelling and ask: is this compelling because the evidence is strong and the reasoning is valid, or because it is pushing a psychological button — fear, loyalty, disgust, or desire for certainty?
AI language models can produce arguments that contain logical fallacies, especially when asked to persuade or when their training data included fallacious reasoning. An AI response that sounds authoritative and well-structured can still commit a straw man, a false dilemma, or a post hoc error. Apply the same fallacy-detection habits to AI output that you apply to human arguments.
A classmate says: 'You cannot take Marco's argument about school lunch seriously — he brings candy bars to class.' Which fallacy is this?
'We should not let students choose any of their own classes, because if we do, they will only pick easy ones and education will completely fall apart.' Which fallacy is present?
'After the city opened a new park, crime in the area dropped 15 percent. The park caused the crime reduction.' What is the problem with this reasoning?
Fallacy Field Guide
- Step 1: Choose three of the six fallacies from this lesson: ad hominem, straw man, false dilemma, appeal to authority, slippery slope, or post hoc.
- Step 2: For each fallacy you chose, write your own original example — a short sentence or two that commits that fallacy. Make the examples realistic (a conversation, a social media comment, an advertisement).
- Step 3: For each example, write one to two sentences explaining exactly which part of the reasoning is flawed and why.
- Step 4: Rewrite one of your fallacious examples as a corrected, non-fallacious argument that makes the same general point without the logical flaw.