copula

In traditional logic a proposition not only contains a subject and a predicate, but also a coupling device or copula (is, are, is not, are not) binding them together. It is generally held that very different things are covered by the notion, such as the ‘is’ of predication (rain is wet), that of identity (Marilyn Monroe is Norma Jean), and that of composition (this statue is marble). The need for a copula arises from a picture in which all the other elements of a proposition are complete ‘terms’, so they need some glue to bind them together into a sentence. Since Frege it has been more common to think of the predicate in a subject–predicate sentence on the model of an expression for a mathematical function, which needs no coupling device to bind it to its argument. No such connection has to be postulated, any more than there is a connection between the square function and 2 in the expression 2².