Existential fallacy

From Wikinfo

The existential fallacy, or existential instantiation, is a logical fallacy committed in a categorical syllogism that is invalid because it has two universal premises and a particular conclusion. In other words, for the conclusion to be true, at least one member of the class must exist, but the premises do not establish this.

Example:

• All inhabitants of other planets are friendly, and Martians are inhabitants of another planet. Therefore, Martians are friendly. (The conclusion assumes there really are some Martians in existence.)
• All unicorns are animals, therefore some animals are unicorns. (This conclusion assumes that unicorns exist.)

The existential fallacy is a syllogistic fallacy. Modern logical constructs, however, allow for conditional logic ("If unicorns existed...")

See also

• Vacuous truth

External links

• Stephen's guide: existential fallacy
• Fallacy files: existential fallacy
• FOLDOC: existential fallacy

v • d • e Formal fallacies Argument from fallacy • Fallacy of modal logic • Masked man fallacy • Appeal to probability • Bare assertion fallacy Fallacy of propositional logic: Affirming a disjunct • Affirming the consequent • False dilemma • Denying the antecedent Fallacy of quantificational logic: Existential fallacy • Illicit Conversion • Proof by example • Quantifier shift Syllogistic fallacy: Affirmative conclusion from a negative premise • Exclusive premises • Necessity • Four-term Fallacy • Illicit major • Illicit minor • Undistributed middle List of fallacies (formal and informal) • Other types of formal fallacy

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.Template:Logic-stub

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