It is a curious feature of logic that statements which refer to the whole of a class do not actually tell us whether there are any members of that class.
All cats are selfish.
(This tells us that if there are such things as cats, then they are selfish. It does not imply that there are cats, any more than the existence of unicorns could be deduced from a similar statement about them.)
Statements which tell us about some of a class, however, do imply the existence of members of the class.
Some cats are selfish.
(This tells us that there are such things as cats, and that some of them are selfish.)
The existential fallacy occurs when we draw a conclusion which implies existence from premises which do not imply that. If our premises are universal telling us about ‘all’ or ‘none’, and our conclusion is a particular one telling us about ‘some’, we have committed the fallacy.
All UFOs are spaceships, and all spaceships are extraterrestrial, so so UFOs are extraterrestrial.
(This seems harmless enough, but it is not valid. We could have said all UFOs were extraterrestrial, but by limiting it to some we imply that they exist.)
It seems puzzling that we can be more entitled to say that all are, than to claim that only some are. We can console ourselves with the thought that perhaps we have to know some of them to start talking about the features which apply to some but not the others. The universal statements, by not sorting any out, carry no such implication.
The fallacy consists of putting into the conclusion something for which no evidence was offered, namely the presumption that what is being talked about actually exists. By going beyond the evidence, we enter into the territory of the fallacy.
All policemen are tall people, and no honest Welshmen are tall people, so some honest Welshmen are not policemen.
(Alas, no evidence has been produced to show that there is such a thing as an honest Welshman.)
A conclusion about all honest Welshmen would have been acceptable, because it would refer only to any who might exist.
The existential fallacy is clearly the domain of those who wish to engage in rational discourse about astral forces and demonic entities but who suffer from the minor disadvantage that there is no evidence that any of these things exist at all. Statements are made telling what the things must be like if they do exist, and somehow we begin to encounter claims made about some of them. At that point, unknown to the audience, the assumption of real existence has been slipped in without evidence, like an ace dropped furtively from the sleeve.
All psychic entities are affected by human emotions, but some of them are more sensitive than others, and tend to be aroused by fear a hatred.
(And the same is true of invisible frogs, spotted Saturnians and warm-hearted Swedes. Before you can start sorting them out, you must first catch your hare.)
Use of the existential fallacy is surprisingly easy. Most audiences will respect your modest claims if you move down from assertions about all things to claims made for only some of them. This readiness is the gate through which you can drive a coach and six loaded with fairies and hobbits, ectoplasm and elementals. The malleability of human nature and the perfectibility of man went through the same gate long ago.
The above article is from the book How To Win Every Argument by Madsen Pirie. The article is only for educational and informative purposes to explain and understand formal logic and logical fallacies. It is a great book, definitely worth a read!