Is has four perfectly distinct meanings in English, besides misuses of the word. Among the misuses, perhaps the most important are those referred to by De Morgan:[72] “... We say ‘murder is death to the perpetrator’ where the copula is brings; ‘two and two are four,’ the copula being ‘have the value of,’ etc.” SchrÖder[73] quite satisfactorily pointed out the well-known distinction between an is where subject and predicate can be interchanged (such as: “the class whose members are Shem, Ham and Japhet is the class of the sons of Noah”) and an is or are where they cannot (such as: Englishmen are Britons), but failed to see[74] the more important distinction (made by Peano) of is in the sense of “is a member of.” If Englishmen are Britons, and Britons are civilized people, it follows that Englishmen are civilized people; but, though the Harmsworth EncyclopÆdia is a member of the class Book (of one or more volumes), and this class is the member of a class A of which it is the only member, yet the Harmsworth EncyclopÆdia is not a member of A, for it is not true that it is the whole class of books; and such a statement would not even be made except possibly in the form of an advertisement. The fourth meaning of is is exists; it is in certain rare moods a matter for regret that there are difficulties in the way of using one word to denote four different things. For, if there were not, we might prove the existence of any thing we please by making it the subject of a proposition, and thereby earn the gratitude of theologians.
CHAPTER XXVII
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