. If it were not equally true in both cases, the reductio ad absurdum would not be a valid method; for the essence of this method consists in using implications in cases where (as it afterwards turns out) the hypothesis is false. We may put the matter another way. In order to understand "all
is
," it is not necessary to be able to enumerate what terms are
's; provided we know what is meant by being an
and what by being a