Fallacies are errors in reasoning that undermine arguments, and they come in two broad kinds. Formal fallacies arise from invalid logical form; informal fallacies arise from problems of content, context, or delivery. Two classic formal fallacies involve conditional arguments: affirming the consequent (if P then Q, Q, therefore P) and denying the antecedent (if P then Q, not P, therefore not Q). In both cases the conclusion does not follow, because Q could be true for reasons other than P, and Q might still be true even when P is false. Other formally invalid moves—those whose conclusions simply do not follow from their premises—are called non sequiturs.
Informal fallacies are far more numerous. Ad hominem attacks the person rather than the argument; straw man misrepresents an opponent's position to refute a distorted version; slippery slope asserts that one event will inevitably trigger a chain of extreme consequences without justifying each link. Several appeals lean on factors other than evidence: appeal to authority (argumentum ad verecundiam) leans on a supposed expert, especially one outside the relevant field; appeal to ignorance (argumentum ad ignorantiam) treats absence of disproof as proof; appeal to emotion manipulates feelings; appeal to nature conflates naturalness with goodness; and the bandwagon fallacy (argumentum ad populum) treats widespread belief as evidence of truth. Genetic fallacy judges claims by their origin; false dilemma presents only two options when more exist; red herring diverts attention with an irrelevant topic; tu quoque deflects criticism by accusing the accuser; circular reasoning assumes the conclusion in its premises (begging the question); and equivocation exploits ambiguous terms.
Hasty generalization leaps from a small or unrepresentative sample to a broad claim; post hoc ergo propter hoc assumes sequence implies causation; the no true Scotsman fallacy excludes counterexamples by redefining the group; loaded question smuggles in an unproven presupposition; composition transfers properties from parts to whole, while division transfers them from whole to parts. The middle ground fallacy (argument to moderation) assumes the truth is a compromise between positions; special pleading applies standards inconsistently to exempt oneself or one's case; the Texas sharpshooter cherry-picks data clusters after the fact; and whataboutism shifts attention to a different issue entirely. Recognizing these patterns is essential for any critical thinker, since fallacious reasoning can be persuasive even when its conclusions are unjustified.