—namely that a single nature does not entail mutually disparate attributes. This is self-evident: existence, in and of itself, does not necessitate either necessity or contingency, just as animality, by itself, does not demand both speech and neighing. Were it so, then wherever existence were present, necessity and contingency would both be present, and one and the same existent would be simultaneously necessary and contingent—yet what is necessary cannot admit non-existence, whereas what is contingent can. That would amount to joining two opposites.
As for the third option—namely, that what is common is only accidental to what differentiates—it would follow that the existence of the Necessary Being is accidental to Him and not essential. There would therefore have to be something of which this necessary existence is an accident. But if the position that His existence is required by something other than Himself is already impossible, then calling it accidental is a fortiori more untenable. An accident, after all, is never sufficient for its own realization; its occurrence depends on a cause external to the essence. Hence His existence would be produced by another and dependent upon something outside His own being—an admission that His existence is contingent, not necessary.
The fourth option is that the differentiating attribute is accidental to what is common; in that event the necessity of the Necessary Being would be an accident of His existence, not intrinsic to it. Yet an accident does not arise from the essence itself; its acquisition relies on an external cause. Thus the necessity of the Necessary Being would depend on something beyond Him: unless that other were realized, He would possess no necessity, and so would not be necessary in Himself but only through another—an evident impossibility.
Accordingly, this line of reasoning is identical to theirs, point for point—indeed, it is even clearer and more cogent. And because it entails denying that reality contains anything necessary in itself or anything contingent, whereas the existence of these two categories is beyond doubt, it follows that this argument