Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

Special consideration is given to opposition and equipollence as they concern modal propositions. “Modal proposition” is taken here as opposed to a proposition de inesse. A proposition de inesse is “one in which it is simply indicated that the predicate belongs and inheres in the subject.” A modal proposition is “one in which it is indicated that the predicate inheres in the subject with the mode in which it inheres and belongs to it.”

Since a mode is nothing other than a determination joined to a thing, as St. Thomas says in the Opusculum on Logic, treatise on Modals,¹ many modes can affect a proposition, but not every mode constitutes the modal proposition with which we are concerned here. Some modes modify only the predicate or subject. For example, if you say, “Peter runs quickly,” or “he acts pleasantly,” the modes “quickly” and “pleasantly” affect only the predicate. And if you say, “A just man is wise,” or “The horse of a man runs,” “just” and “of a man” modify, that is, restrict the subject; but they do not remove the proposition from the account of a proposition de inesse, such that it would otherwise have different rules of opposition or equipollence.

Other modes affect the copula itself and the predicate’s inherence in the subject, and, by modifying it, add something pertaining to universality or particularity, or to affirmation and negation. These modes make the modal propositions with which we are concerned here; there are four: impossible, possible, necessary, and contingent. Some add two others, true and false. But these are not properly modes of the kind with which we are concerned here, because truth and falsity follow every proposition in such a way that they do not alter the order and mode of opposition and equipollence, etc., found in propositions de inesse.

A modal proposition is therefore defined as “one that is affected by one of these four modes.” It is twofold, since it can be affected in two ways: one modal proposition is divided, the other composite. A composite modal proposition is one in which the mode functions as predicate and the dictum itself as subject; for example, “That Peter disputes is possible.” A divided modal proposition is one in which the mode, taken adverbially, is applied to the copula, as in “Peter possibly disputes.”

Note that this is the origin of the composite and divided senses in propositions: the composite sense is taken from a composite modal proposition, and the divided sense from a divided one. These senses consist in this: in the composite sense the simultaneity and union of two forms in one subject is signified; in the divided sense the union or inherence of two forms in a subject is signified not simultaneously but successively, with one excluding the other; this is what “divided” means. Hence a transition from a divided modal proposition to a composite one, or from a divided sense to a composite one, is never valid, just as a transition from successive inherence to simultaneous inherence is not valid.

The composite sense is therefore well signified by a composite modal proposition, in which the mode is predicated of the whole dictum; for it signifies that the form of the predicate and that of the subject are united and belong together under that mode. The divided sense, however, is signified by a divided modal proposition, in which the mode affects only the copula and thus indicates that the mode belongs to the subject, but does not indicate that the forms of predicate and subject belong together simultaneously. For example, “For black to be white is possible,” and “For one sitting to stand is possible”: in the composite sense this signifies that the conjunction of sitting and standing is possible. But if you say, “One sitting can stand,” or “is able to stand,” in the divided sense, it signifies that the power to stand belongs to the subject while sitting, but not the power to stand simultaneously with sitting. Thus, when opposed forms are signified in the subject and predicate—because one form does not exclude the subject’s capacity to receive another, but only its co-existence and simultaneity with that form—the capacity remains in the divided sense while that form remains in the subject; for that form neither removes the capacity nor posits it. Accordingly, when I say, “A will moved efficaciously can fail to act,” in the divided sense, I do not mean that by dividing efficacious motion it can fail to act. Rather, while the efficacious motion remains in the subject, the capacity for the opposite remains, but not the capacity to unite the opposite with the form of efficacious motion. Thus the divided sense posits in the subject a capacity for opposites successively, whereas the composite sense indicates a capacity for opposites conjoined simultaneously.