differ in their individual instances yet concur in a property that constitutes them must, they say, be such that the element on which they agree is either the cause of that in which they differ, or its effect, or an accident of it, or the subject that carries that accident. This is a mistake, for the prior element in which they are said to agree can be neither an accident nor a subject of one: according to their own principles, whatever constitutes an essence is never an accident and never a substrate. What they should have said is simply, “The things that differ in their individual realities while agreeing in some respect….” All this, however, is no more than criticism of the outward form of the proof, drawing attention to the padding and the turns of phrase that ought never to have been inserted.

Second: Concerning their assertion, “If the two differ with respect to existence and necessity, then there are not two Necessary Existents,” we reply: you have treated difference in individual realities as numerical distinction only; you did not intend (1) by “difference” that one should exist and the other be nonexistent. Rather, whenever two things share in one aspect and diverge in another, you call the aspect in which they diverge their “difference.” Thus, if they are distinct in their individual realities, they are different in their individual realities. When, therefore, we say that they differ in existence and necessity, we mean that they differ in a particular existence and a particular necessity: this determinate existence and necessity belong to one, and are not the determinate existence and necessity of the other. Accordingly, if it is said that what they share—namely, in kind—is accidental to that in which they differ—namely, in individuation—the true sense of the statement is that the common element is accidental to the distinguishing element; and this is a firmly established truth for every species.

Why, then, did you declare that a determinate existence and a determinate necessity cannot admit of what the two share, such as an unrestricted quiddity (2), an absolute necessity of existence, and the like? On the contrary, that is precisely the case, for the existence of concrete particulars in the external world does not of itself entail the existence of the shared universal qua universal; that universality is conditioned by its locus, namely the mind.