Definition · Plain-language
False analogy
A false analogy is an informal logical fallacy in which two things are compared as though analogous, when they are not relevantly similar in the way the argument requires.
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Structure of analogical arguments and the relevance criterion
An analogical argument has the form: A and B share properties P1, P2 … Pn; A also has property Q; therefore B probably has property Q too. The strength of the analogy depends on whether the shared properties (P1…Pn) are relevant to Q. A false analogy fails the relevance criterion: the shared properties are irrelevant to the conclusion, or the cases differ in a property that is highly relevant. Identifying whether similarities are relevant — and whether differences are material — is the core analytical task in evaluating an analogy. The more properties the two cases share that are relevant to the conclusion, and the fewer relevant differences they have, the stronger the analogy.
False analogies in politics, ethics and law
Political and ethical analogies are particularly prone to being false. Comparing a modern government’s policy to Nazi Germany ("reductio ad Hitlerum") typically fails because the relevant respects differ enormously in scale, mechanism and intention. In law, analogical reasoning from precedent is fundamental — a new case is decided by analogy to prior cases — but a false legal analogy occurs when the current case differs from the precedent in legally relevant ways. English law developed the "ratio decidendi" (the legally relevant reason for a decision) partly to distinguish it from the "obiter dicta" (remarks not materially relevant to the outcome), so that precedent is applied only to relevantly similar cases.
Evaluating analogies: strong versus weak
To evaluate an analogy, ask: (1) In what respects are the two things similar? (2) Are those similarities relevant to the conclusion? (3) In what respects do the two things differ? (4) Are those differences relevant to the conclusion? A strong analogy has many relevant similarities, few relevant differences, and the conclusion depends on the shared relevant properties. A weak analogy has few relevant similarities or many relevant differences in properties material to the conclusion. The disanalogy test — actively searching for relevant differences — is the practical counter to being misled by superficially compelling comparisons.
Key facts
At a glance
- Definition: comparing two things as analogous when not relevantly similar in the required way
- Type: informal fallacy of weak induction
- Relevance criterion: shared properties must be relevant to the conclusion
- Legal analogy: ratio decidendi ensures precedent applies to relevantly similar cases
- Evaluation: relevant similarities vs relevant differences
- Strong analogy: many relevant similarities, few relevant differences
- Disanalogy test: actively search for relevant differences to evaluate the analogy
Common misconceptions
What people often get wrong
Often heard: All analogies are false analogies and should be avoided.
Actually: Analogical reasoning is a legitimate and powerful form of inference used throughout science, law and ethics. A false analogy is one that fails the relevance test. Well-constructed analogies with relevant shared properties and few relevant differences are valuable tools of thought.
Often heard: If two things share some similarities, the analogy is valid.
Actually: The number of similarities is less important than their relevance to the conclusion. Two objects can share dozens of irrelevant properties while differing fundamentally in the property the argument requires. Relevance of the shared properties is what matters.
Often heard: A false analogy is always deliberate.
Actually: False analogies are often committed in good faith by people who find two situations genuinely similar without carefully analysing which similarities are relevant to the specific conclusion. The fallacy is defined by the logical failure, not by intent.
Common questions
FAQ
What makes an analogy false?+
An analogy is false when the two things compared differ in properties that are relevant to the conclusion the argument is drawing. Similarities that are irrelevant to the conclusion do not support it, however numerous. The key question is always whether the similarities are the right kind — materially related to the point being argued.
How is analogical reasoning used legitimately in law?+
Common-law systems reason from precedent by analogy: a new case is decided by reference to prior cases with relevantly similar facts and legal issues. The "ratio decidendi" — the legal principle on which a case was decided — is extracted and applied to later cases sharing the relevant similarities. Skill in legal reasoning involves identifying when the analogy holds and when differences are material enough to distinguish the cases.
How do I evaluate an analogy I encounter?+
Work through four questions: (1) What properties do the two cases share? (2) Are those shared properties relevant to the conclusion? (3) In what ways do the two cases differ? (4) Are those differences relevant to the conclusion? A strong analogy has many relevant similarities and few relevant differences. Systematically searching for relevant differences — the disanalogy test — is the best practical defence against being misled.
Going deeper
