In using names of these different kinds; names of individuals when the idea is restricted to one individual; and, for brevity, the names of classes; the names of the less when necessary, of the large when practicable; there is perpetual need of the substitution of one name for another. When I have used the names, James and John, Thomas and William, and many more, having to speak of such peculiarities of each, as distinguish him from every other, I may proceed to speak of them in general, as included in a class. When this happens, I have occasion for the name of the class, and to substitute the name of the class, for the names of the individuals. By what contrivance is this performed? I have the name of the individual, _John_; and the name of the class _man_; and I can set down my two names; _John_, _man_; in juxta-position. But this is not sufficient to effect the communication I desire; namely, that the word man is a mark of the same idea of which John is a mark, and a mark of other ideas along with it, those to wit, of which James, Thomas, &c. are marks. To complete my contrivance, I invent a mark, which, placed between my marks, {161} _John_ and _man_, fixes the idea I mean to convey, that _man_, is another mark to that idea of which _John_ is a mark, while it is a mark of the other ideas, of which _James_, _Thomas_, &c., are marks. For this purpose, we use in English, the mark "is." By help of this, my object is immediately attained. I say, _John_ "is" a _man_. I, then, use the word _man_, instead of the word _John_, with many advantages; because every thing which I can affirm of the word _man_, is true not only of _John_, but of _James_, and _Peter_, and every other individual of the class. The joining of two names by this peculiar mark is the act which has been denominated, PREDICATION; and it is the grand contrivance by which the marks of sensations and ideas are so ordered in discourse, as to mark the order of the trains, which it is our purpose to communicate, or to record. The form of expression, "John is a man," is called a Proposition. It consists of three marks. Of these, "John," is denominated the SUBJECT; "man," the PREDICATE; and "is," the COPULA. To speak generally, and in the language of the grammarians, the nominative of the verb is the _subject_ of the proposition; the substantive, or adjective, which agrees with the nominative, is the _predicate_, and the verb is the _copula_. By a few simple examples, the reader may render familiar to himself the use of PREDICATION, as the grand expedient, by which language is enabled to mark not only sensations and ideas, but also the order of them.[48] [Editor's footnote 48: The theory of Predication here set forth, stands in need of further elucidation, and perhaps of some correction and addition.