The English "to be", and its equivalents in certain other languages, also have a non-copular use as an existential verb, meaning "to exist." This use is illustrated in the following sentences: "I want only to be, and that is enough"; "I think therefore I am; To be or not to be, that is the question." In these cases, the verb itself expresses a predicate (that of existence), rather than linking to a predicative expression as it does when used as a copula. In ontology it is sometimes suggested that the "is" of existence is reducible to the "is" of property attribution or class membership; to be, Aristotle held, is to be "something". However, Abelard in his "Dialectica" made a "reductio ad absurdum" argument against the idea that the copula can express existence.