then one of them would possess both existence and necessity, while the other would possess only existence or only necessity. That is precisely the conclusion required: there cannot be two Necessary Existents.
If, instead, they were thought to differ in their essences while agreeing in existence and necessity, this too is untenable, because—as has already been shown—neither an essence nor any of its attributes can be the cause of existence, for the two impossibilities cited earlier would follow.
Second. The point on which they differ might be a concomitant of the point on which they agree. This alternative has already been dismissed, for a single nature does not entail mutually divergent concomitants.
Third. The point on which they agree might be an accident of the point on which they differ, so that some further attribute accrues to both; they would then diverge over yet another attribute while existence and necessity attach to them. If they diverge with respect to existence and necessity, there would no longer be two Necessary Existents, which contradicts the hypothesis. If they agree in existence and necessity and another attribute supervenes, the cause of that attribute must be either existence and necessity themselves or something outside them. Existence cannot be the cause of such a contingent attribute, for a single principle cannot give rise to disparate effects—this occurs only in composite individuals. Nor can its cause be something other than existence and necessity, for that would make the Necessary Existent dependent on something else, which is impossible, since necessary existence is by nature independent of anything beyond itself.
Fourth. Finally, the point on which they differ might be an accident wholly separate from the point on which they agree. Although this is conceivable in abstract terms, it is inadmissible here, for the accident in question is either existence and necessity themselves or something else. If existence and necessity are taken as a detachable accident, the position is absurd, for necessary existence is incompatible with such a characterization.