in the way in which we find objects separated in that way they are distinct from each other. If, therefore, they are separated only in thought then they are rationally distinct; if they are really separated, they are really distinct. And although there is some doubt whether those objects which are really distinct are also really separable, there cannot be a doubt that those which are really separate are really distinct. If, indeed, an essence were always joined to an existence, it would always possess an existence and it would never be able not to exist.
But it seems to me this very criterion could be used to argue against the "real distinction". According to the separability criterion, two things are really distinct if at least one of them can exist without the other. So according to the "real distinction", either a thing's essence or its existence can "exist" without the other, or both. And Giles argues (as Thomas does) that an essence is not always joined to an existence, ergo etc. But what else is this than to say that the essence "exists" without existence, either in the divine mind or in a created intellect? But if the essence exists there, is it really separated from existence? It seems not. Now, perhaps we don't want to call the kind of being an essence has in the mind existence. This seems right. But then there also seems to be reason to question whether the essence as it "exists" in the mind and as it exists in reality are numerically the same. Arbority in the mind and arbority as it exists as a formal component of this existent oak tree have the same formal ratio, that is, share in the same common nature; to that extent they are identical; but that formal ratio undergoes no change when the oak tree is created. It's not a subject which is actualized. The tree-ish act is not the act of the common nature, but the act of the substance, namely, this individual form united to this matter. And before the tree comes into existence this form, which qua arbority shares the same common nature with the concept in the mind, has no separate being. Since the act of existence is the act of a real singular form, and since the form which exists in the mind is identical with the real singular form only specifically and not numerically, it seems that before the singular form exists in act it is simply nothing. Therefore it is never separated from existence, and therefore it is not really distinct from it.
I have to admit that I am none too sure about this argument. As a matter of fact it seems more problematic for Scotus' metaphysics than for Thomas'. Since Thomas does not admit that there is a singular form in the same sense that Thomas does, it's easier to say that the universal essence is not identical with the form as actualized by the singular existence. Since there is no "Socrateity" in the mind of God we don't have to worry about the unique singularity of Socrates' essence. For Scotus, however, there is such a singular form, and so it would be easier to argue that Socrateity as distinct from Socrates' existence is present in the mind. The question I suppose is whether the ratio of the pre-created Socrateity in the divine mind is numerically identical with Socrateity as existing in Socrates. This bears more thinking about, since I'm not yet sure what to think.