74 How to Win Every Argument
You can add extensional pruning to your repertoire once you
are adept at making a limited statement pass itself off as a wider
one. Gather yourself a collection of phrases whose meaning is
understood by everyone, even though the words themselves are
more restricted.
/ said I'd get you another drink if I was wrong: water is another drink.
I said I wouldn't have any more cigarettes until later in the week. Five
minutes afterwards was later in the week.
(Speak softly, and carry a big dictionary.)False conversion
False conversion takes place when we deduce from the fact that
all cats are animals the additional information that all animals are
cats. The converse of a statement, made by exchanging the
subject and predicate, is true in some cases, false in others. When
it is performed for one of the invalid cases, it is called false
conversion.
All rats are four-legged animals, so obviously all four-legged animals are
rats.
(This one is obviously false. Others are less so.)Some mortal beings are not cats, therefore some cats are not mortal
beings.
(It would be remarkable if the existence of beings other than cats
were sufficient to establish the existence of an immortal strain of
cats.)The rule is intricate, but worth learning. We can make state-
ments about all or some, and we can make positive or negative
assertions. This gives us four types of statement: