Illicit major

Formal fallacy

Illicit major is a formal fallacy committed in a categorical syllogism that is invalid because its major term is undistributed in the major premise but distributed in the conclusion.

This fallacy has the following argument form:

  1. All A are B
  2. No C are A
  3. Therefore, no C are B

Example:

  1. All dogs are mammals
  2. No cats are dogs
  3. Therefore, no cats are mammals

In this argument, the major term is "mammals". This is distributed in the conclusion (the last statement) because we are making a claim about a property of all mammals: that they are not cats. However, it is not distributed in the major premise (the first statement) where we are only talking about a property of some mammals: Only some mammals are dogs.

The error is in assuming that the converse of the first statement (that all mammals are dogs) is also true.

However, an argument in the following form differs from the above, and is valid (Camestres):

  1. All A are B
  2. No B are C
  3. Therefore, no C are A

See also

  • Illicit minor
  • Syllogistic fallacy

External links

  • Illicit Major The Fallacy Files
This article was originally based on material from the Free On-line Dictionary of Philosophy, which is licensed under the GFDL.


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Common fallacies (list)
Formal
In propositional logic
  • Affirming a disjunct
  • Affirming the consequent
  • Denying the antecedent
  • Argument from fallacy
  • Masked man
  • Mathematical fallacy
In quantificational logic
Syllogistic fallacy
Informal
Equivocation
Question-begging
Correlative-based
Illicit transference
Secundum quid
Faulty generalization
Ambiguity
Questionable cause
Appeals
Consequences
Emotion
Genetic fallacy
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of relevance
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