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Existential fallacy

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Wikipedia

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 all Martians are inhabitants of another planet. Therefore, some 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...")