68 How to Win Every Argument
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 some
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