The Principles of Mathematics (1903)

§ 43

It has always been customary to divide propositions into subject and predicate; but this division has the defect of omitting the verb. It is true that a graceful concession is sometimes made by loose talk about the copula, but the verb deserves far more respect han is thus paid to it. We may say, broadly, that every proposition may be divided, some in only one way, some in several ways, into a term (the subject) and something which is said about the subject, which something I shall call the assertion. Thus Socrates is a man may be divided into Socrates and is a man. The verb, which is the distinguishing mark of propositions, remains with the assertion; but the assertion itself, being robbed of its subject, is neither true nor false. In logical discussions, the notion of assertion often occurs, but as the word proposition is used for it, it does not obtain separate consideration. Consider, for example, the best statement of the identity of indiscernibles: If x and y be any two diverse entities, some assertion holds of x which does not hold of y. But for the word assertion, which would ordinarily be replaced by proposition, this statement is one which would commonly pass unchallenged. Again, it might be said: Socrates was a philosopher, and the same is true of Plato. Such statements require the analysis of a proposition into an assertion and a subject, in order that there may be something identical which can be said to be affirmed of two subjects.(§ 43 ¶ 1)