Logical Fallacies
Formal vs Informal Fallacies
Learn to distinguish between arguments that fail because of broken logical structure and those that fail because of misleading content or irrelevant premises. This distinction is crucial because the two types require different diagnostic strategies — formal fallacies can be detected by examining abstract structure alone, while informal fallacies require evaluating the relevance and truth of the content.
Context
Why this exercise
Fallacies fail in two structurally different ways, and confusing them is one of the most common mistakes in argument analysis. Formal fallacies fail because the logical structure is invalid: 'All dogs are mammals; all cats are mammals; therefore all cats are dogs' is broken regardless of the subject matter, and any argument with that shape fails for the same reason. Informal fallacies fail because of content — irrelevant premises, ambiguous terms, hidden presuppositions, emotional manipulation. The two types require different diagnostic strategies, and this exercise drills the distinction by asking you to identify which kind of defect is doing the damage in each argument.
Before you start
The formal versus informal distinction has been central to logic since Aristotle's Prior Analytics (formal syllogistic) and Sophistical Refutations (informal fallacies). Formal logic, developed further through medieval logicians and culminating in Frege, Russell, and modern symbolic logic, evaluates arguments by abstract structure: if you replace the subject terms with variables, an invalid form remains invalid regardless of what you plug in. Affirming the consequent (if P then Q; Q; therefore P) and denying the antecedent (if P then Q; not P; therefore not Q) are the two most common formal fallacies, and both look superficially similar to the valid forms modus ponens and modus tollens. The undistributed middle, illustrated in the cats-dogs-mammals example, is another classic formal failure: 'all A are B; all C are B; therefore all C are A' is invalid because B (mammals) is the middle term and it is undistributed in both premises.
Informal fallacies do not have a single unifying structural diagnosis. They are a heterogeneous collection of failure modes, traditionally grouped into families: relevance fallacies (the premise is true but not relevant to the conclusion — ad hominem, red herring, appeal to emotion), presumption fallacies (the argument presupposes what it is trying to prove — begging the question, complex question, false dilemma), and ambiguity fallacies (the argument exploits unclear or shifting meanings — equivocation, amphiboly). Douglas Walton's argumentation schemes give a more systematic modern treatment that handles cases the classical taxonomy struggles with, but for everyday use the family-based grouping is sufficient.
The practical payoff of the distinction is in how you respond. Formal fallacies are defused by showing the structure: substitute different terms and demonstrate that the conclusion still does not follow, which makes the structural defect undeniable. Informal fallacies are defused by addressing the content: showing that the premise is irrelevant, or that an unstated assumption is doing the real work, or that a key term has shifted meaning between premise and conclusion. The wrong-answer options in this exercise frequently misclassify the type — calling an informal fallacy formal or vice versa — and discriminating between them is exactly what the scenarios train. For deeper treatment, see Logical Fallacies: Formal & Relevance.