A fallacy, as the term is usually employed, describes a common misconception such as “Handling frogs causes warts”, but more formally it characterises the reasoning that leads to the belief in such a statement.[1]
The existential fallacy has its roots in the principle of Aristotelian logic called dictum de omni, which says that everything said universally about a subject is true about everything contained under that subject.[2] It is one of the commonest fallacies in Western philosophy,[3] and occurs when it is erroneously supposed that some class or group has members. In other words, that statements may be true about classes or groups even if there are no members of those classes or groups.[4]
For example, it would be true to conclude from the claim that “All humans are mortal” that “Some humans are mortal”. But it would be false to conclude from the claim that “All trespassers will be prosecuted” that “Some of those prosecuted will have trespassed”, as the set of trespassers may be empty; those prosecuted may have committed other offences, not trespass. In other words, it is impossible to reason about empty classes.[5]
The philosopher Immanuel Kant, on the other hand, argued that the truths of semantic logic do not require the existence of the entities to which the truths refer.[2]