The existential fallacy 67
(Many of us are negative about puddings, but two negative state-
ments about them will not tell us anything about smokers. If smokers
are thin, it might well be from worrying about the health warnings,
and a lack of money to eat puddings after paying for the cigarettes.)When you want to use the fallacy of exclusive premises, you
should try to make your negative statements ones which the
audience will accept the truth of. When you slip in a conclusion
which seems plausible, they will assume that you have proved it.
You will not get very far if you start out with statements such as
'No council workers are lazy', but should try instead to keep
within the bounds of your audience's experience. Use obvious
truisms such as 'No removal men are careful.'
The existential fallacy
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 //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.)