A proposition is an assertion of something about something. More clearly, a proposition articulates a relation between a subject and a predicate. A proposition, then, consists of three distinct but inseparable elements, viz. (i.)the Subject Term (ST), (ii.)the Copula (C), and (iii.)the Predicate Term (PT). In everyday expression, our assertions are either affirmative or negative. The proposition “John is good,” for instance, is an affirmative assertion; the proposition in question affirms that John is identifiable as good. The negation of the goodness of John would be the proposition “John is not good,” denying that John is identifiable as good.
Ontologically, however, the situation seems to be different. For example, I may deny that p, where p is the proposition “John is good,” but the resulting proposition ￢p, viz. “John is not good,” would be logically equivalent to affirmative proposition, p1: “John is a member of the class of not-good things.” Ontologically, therefore, the proposition seems to be essentially affirmative.
The ST-PT relation is a relation of identity between a distinct ST and a distinct PT. For example, the proposition “John is good ” identifies goodness and John. More elaborately, the proposition means something like “John is a person who performs good actions.” The relation of identity is modal, therefore, relating an ontological John with some distinct demonstration of his ontology.
John as the ST, and John as the implicit ST of the PT, therefore, is essentially identical, but differs with respect to the demonstration of his goodness.
One in essence, yet three in distinct members, the proposition reflects the unity and difference in the Trinity. As the Father is explicated by the Son, so the subject is explicated by the predicate. As the Spirit of God reveals the Father-Son relationship, so the copula reveals the subject-predicate relationship. As the Godhead is inextricably One in essence, so too the proposition is inextricably one.