Existential fallacy

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...")