Affirming a disjunct

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Affirming a disjunct is a form of logical fallacy where an alternative possibility is rejected because the first is accepted, even if both are accurate. The problem with this fallacy is typically an underlying misunderstanding that establishes false mutual exclusivity and/or a false dichotomy.

Formal Construction[edit]

  • A or B
  • A
  • Therefore, it's not B


  • Evolution
  1. Evolution is either a theory or a fact
  2. Evolution is a theory
  3. Thus, Evolution is not a fact
  • Choice of Religion
  1. Either God is the Muslim god or he is the Christian god.
  2. God is the Christian god
  3. Thus, God is not the Muslim god
  • Choice of God or Evolution
  1. Either God created animals or they evolved over time
  2. God created animals
  3. Thus, Evolution is false

Ways to Avoid Affirming a Disjunct[edit]

Make sure that the two initial possibilities are in fact mutually exclusive. Try to think of ways that both - or neither - could be true, and thus ways that the argument could be invalid.

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