Affirming the consequent

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Affirming the consequent, sometimes also called asserting the consequent or the converse error, is a type of logical fallacy where a premise is asserted as true simply because a conclusion implied by the premise is true. This is a fallacy because it assumes that the conclusion could only have been reached in one particular way.

For example:

  1. If an animal is a rat, then it has four legs
  2. My dog Lassie has four legs
  3. Therefore, Lassie is a rat

Formal Construction

  • A implies B
  • B
  • Therefore, A

Further Examples

  • Creationism example 1
  1. If the universe was created by God, then there would be order and natural laws observed in the universe
  2. We see examples of order and natural laws in the universe
  3. Therefore, the universe was created by God

This argument assumes that a deity is the one and only explanation for order in the universe. And even if we do assume that a deity is the only explanation, it doesn't automatically mean that it must be the Christian deity. The Flying Spaghetti Monster is a good counter-example that satirically exposes the underlying fallacy of this argument.

  • Creationism example 2
  1. If the Biblical flood story is true, then there was a boat built with the dimensions described in the Bible
  2. We found a boat with these dimensions
  3. Therefore, the Biblical flood story is true.

To see why this is a fallacy, consider the same logic applied to the story of Cinderella and the existence of Cinderella's castle in Disneyland.

See Also

v · d Logical fallacies
v · d Formal fallacies
Propositional logic   Affirming a disjunct · Affirming the consequent · Argument from fallacy · False dilemma · Denying the antecedent
Quantificational logic   Existential fallacy · Illicit conversion · Proof by example · Quantifier shift
Syllogistic   Affirmative conclusion from a negative premise · Exclusive premises · Necessity · Four-term fallacy · Illicit major · Illicit minor · Undistributed middle

v · d Faulty generalisations
General   Begging the question · Gambler's fallacy · Slippery slope · Equivocation · argumentum verbosium
Distribution fallacies   Fallacy of composition · Fallacy of division
Data mining   Cherry picking · Accident fallacy · Spotlight fallacy · Hasty generalization · Special pleading
Causation fallacies   Post hoc ergo propter hoc · Retrospective determinism · Suppressed correlative · Wrong direction
Ontological fallacies   Fallacy of reification · Pathetic fallacy · Loki's Wager
v · d False relevance
Appeals   Appeal to authority · Appeal to consequences · Appeal to emotion · Appeal to motive · Appeal to novelty · Appeal to tradition · Appeal to pity · Appeal to popularity · Appeal to poverty · Appeal to spite · Appeal to wealth · Sentimental fallacy · Argumentum ad baculum
Ad hominem   Ad hominem abusive · Reductio ad Hitlerum · Judgmental language · Straw man · Tu quoque · Poisoning the well
Genetic Fallacies   Genetic fallacy · Association fallacy · Appeal to tradition · Texas sharpshooter fallacy
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