Edwards sets out to explain in this chapter the definitions of the modal terms he employs in his work; namely, "necessity" and "possibility." He first broaches the definition of 'necessity' as that which must be, or, to put it negatively, that which cannot but be, or which cannot be otherwise. He rejects this definition because he notes that each of the words employed in this definition must themselves be defined. Its use in ordinary speech, he pointst out, is "relative." Something is necessary in relation to another thing. There is some thing which is impossible to overcome in its course or being, and such a thing we say is 'necessary.' Furthermore, he notes, something is "necessary" to our personal wills when we cannot but do such and such a thing, exert such and such an effort, have such and such a desire, and so on. Likewise, something is impossible to do if no exertion of ours can bring it to pass because of some irresistible opposition to our end.
Edwards notes that philosophers sometimes use words such as necessary or possible in highly technical and unconventional senses, like with reference to the existence of God. In these cases, "necessity" refers simply to "certainty." No reference, he notes, is made to the existence to the will. Instead, metaphysical necessity is being referred to in such cases. Edwards defines this sort of philosophical necessity as "the FULL AND FIXED CONNECTION BETWEEN THE THINGS SIGNIFIED BY THE SUBJECT AND PREDICATE OF A PROPOSITION"(Edwards). When a subject and predicate have a "full and CERTAIN CONNECTION" then necessity obtains in a metaphysical or necessary sense.
This may obtain because for something not to be in a certain way would entail an absurdity. For example, a square has four sides. To suppose that there is such a thing as a square circle is necessarily absurd, because having four sides inheres in a square. God likewise has many attributes which are necessary, to use an example more familiar to Christians. It is impossible for God to lie (Heb. 6:18) precisely because it is a necessary component of God's being that He is truth(Jhn. 14:6).
(2.) The connection of the subject and predicate of a proposition, which affirms the existence of something, may be fixed and made certain, because the existence of that thing is already come to pass; and either now is, or has been; and so has, as it were, made sure of existence. And therefore, the proposition which affirms present and past existence of it, may by this means be made certain and necessarily and unalterably true; the past event has fixed and decided the matter, as to its existence; and has made it impossible but that existence should be truly predicated of it. Thus the existence of whatever is already come to pass, is now become necessary; it is become impossible it should be otherwise than true, that such a thing has been. We may also speak of analytic necessity, as in the case of the proposition "all bachelors are unmarried males", where the predicate inheres in the subject.
Edwards also speaks of a kind of causal necessity, according to which a future event is "necessary" because of causal antecedence of some other event which makes it such that it is impossible but that it eventually be brought about. Indeed, for the Calvinist, all events, past, present and future, are necessary, precisely because of that attribute of God which is itself necessary, namely, His sovereignty, according to which His absolute predestination takes place.
Edwards makes a useful distinction between general and particular necessity:
General necessity - something whose existence is such that some antecedent foundation acts as the irresistible cause of its existence. General necessity also obtains when a predicate inheres in a subject within a proposition.
Particular necessity - When it is certain that such and such an event will come about because of the impossibility of opposition to it.