A proposition, we have before said, is a portion of discourse in which a predicate is affirmed or denied of a subject. A predicate and a subject are all that is necessarily required to make up a proposition: but as we can not conclude from merely seeing two names put together, that they are a predicate and a subject, that is, that one of them is intended to be affirmed or denied of the other, it is necessary that there should be some mode or form of indicating that such is the intention; some sign to distinguish a predication from any other kind of discourse. This is sometimes done by a slight alteration of one of the words, called an _inflection_; as when we say, Fire burns; the change of the second word from _burn_ to _burns_ showing that we mean to affirm the predicate burn of the subject fire. But this function is more commonly fulfilled by the word _is_, when an affirmation is intended, _is not_, when a negation; or by some other part of the verb _to be_. The word which thus serves the purpose of a sign of predication is called, as we formerly observed, the _copula_. It is important that there should be no indistinctness in our conception of the nature and office of the copula; for confused notions respecting it are among the causes which have spread mysticism over the field of logic, and perverted its speculations into logomachies. It is apt to be supposed that the copula is something more than a mere sign of predication; that it also signifies existence. In the proposition, Socrates is just, it may seem to be implied not only that the quality _just_ can be affirmed of Socrates, but moreover that Socrates _is_, that is to say, exists. This, however, only shows that there is an ambiguity in the word _is_; a word which not only performs the function of the copula in affirmations, but has also a meaning of its own, in virtue of which it may itself be made the predicate of a proposition. That the employment of it as a copula does not necessarily include the affirmation of existence, appears from such a proposition as this, A centaur is a fiction of the poets; where it can not possibly be implied that a centaur exists, since the proposition itself expressly asserts that the thing has no real existence.