The existential fallacy, or existential instantiation, is a formal fallacy, where we presuppose that a class has members when we are not supposed to do so; that is, when we should not assume existential import.
Every unicorn definitely has a horn on its forehead.
It does not imply that there are any unicorns at all in the world, and thus it cannot be assumed that, if the statement is true, somewhere there is a unicorn in the world (with a horn on its forehead). The statement, if assumed true, only implies that if there were any unicorns, each would definitely have a horn on its forehead.
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