in itself does not admit non-existence; for if at one time it accepted non-being and at another being, it would be contingent. These temporal events, therefore, are not impossible—because they have in fact occurred—nor are they necessary, for they were once absent. It follows that they are possible, receptive of both existence and non-existence. Hence it is known with certainty that among existing things there is that which is necessary in itself, and there is that which is possible in itself: it possesses no existence of its own but exists through another—that is the contingent being.

We therefore say: these two kinds of being—the one necessary in itself and the one contingent in itself, whose existence is necessary through another—share the common designation “existence,” while each is distinguished from the other by necessity or contingency. The point on which they agree must be related to the point on which they differ in one of four ways: what they share is either consequent upon what distinguishes them, or it necessitates it, or it is an accident of it, or its subject.

If the first alternative is adopted, so that existence is taken to be consequent upon their difference, then, since the disagreement would lie in what they actually share—namely existence—it would follow that one of the two exists while the other does not. But if the disagreement is said to lie in what separates them, namely necessity and contingency, while their agreement is in existence—which is the true division—then what distinguishes them would be the cause of what they share. In that case necessity and contingency would be the cause of existence; yet a cause precedes its effect in existence, which would entail that necessity precedes existence in existence itself. Thus a thing would exist before it exists, or it would exist twice over—exactly the dilemma they acknowledge in speaking of quiddity alongside existence. The reasoning concerning necessity with existence is therefore the same as their reasoning about quiddity with existence.

Still graver absurdities follow. If the sheer possibility of a contingent being were the cause of its existence, contingent beings would no longer be in need of the Necessary; the contingent would, in itself, be necessary in itself, for its own possibility would be both its cause and its existence. Indeed, every conceivable contingent would then exist. Consequently, if what differentiates the two is made the cause of what they share, the result is a cluster of impossibilities that rank among the most egregious errors.

Should one opt for the second alternative—that what distinguishes them is consequent upon what they share—its falsity has already been exposed in their own discussion through what was previously stated about the nature