The existential fallacy (aka instantiation fallacy) occurs when a universal generalization is used to conclude that some (or at least one) instance of that universal class actually exists. This overlooks that a universal statement may only be defining what the concept entails without implying the actual existence of anything matching that concept.

Here is an example:

1. Magic genies can pop out of a bottle and then grant someone three wishes.

2. Therefore, there are magic genies.

The error is that the first statement describes only what magic genies would do if they existed, and does not say whether or not they do in fact exist. It means, effectively, "Anything that is a magic genie, can pop out of a bottle, etc.," which leaves open the possibility that nothing is in fact a magic genie.

This fallacy does not pertain only to mythical creatures, as demonstrated in the following example:

1. The corner drugstore has a sign that says "All shoplifters are prosecuted to the fullest extent of the law."

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2. Therefore, there have been shoplifters at the corner drugstore.

It is invalid to infer the second statement from the first, because the sign may have been put up just to warn any potential shoplifters; there might not have been any shoplifters yet in the drugstore.