"Thus," he goes on "if we compare the thoughts _water_, _iron_, and _rusting_, we find them congruent, and connect them into a single thought, thus: _water rusts iron_--in that case we form a judgment." When a judgment is expressed in words, he says, it is called a proposition.
But has a proposition no meaning beyond the judgment it expresses?
Mill, who defines it as "a portion of discourse in which a predicate is affirmed or denied of a subject" (_Logic_, Book 1., chap.iv.Sec.

1.), proceeds to inquire into the import of propositions (Book 1., chap.

v.), and finds three classes of them: (a) those in which one proper name is predicated of another; and of these Hobbes's Nominalist definition is adequate, namely, that a proposition asserts or denies that the predicate is a name for the same thing as the subject, as _Tully is Cicero_.
(b) Propositions in which the predicate means a part (or the whole) of what the subject means, as _Horses are animals_, _Man is a rational animal_.

These are Verbal Propositions (see below: chap.v.Sec.

6), and their import consists in affirming or denying a coincidence between the meanings of names, as _The meaning of 'animal' is part of the meaning of 'horse.'_ They are partial or complete definitions.
But (c) there are also Real Propositions, whose predicates do not mean the same as their subjects, and whose import consists in affirming or denying one of five different kinds of matter of fact: (1) That the subject exists, or does not; as if we say _The bison exists_, _The great auk is extinct_.