Theorem 12: "Everything that exists, except the First Being, is not its own existence, but it has an essence really distinct from existence and by reason of the former it is a being and by reason of the latter it is an existant."
[...]
"We ought to note, nevertheless, that some objects can be separated actually; others only in thought. Therefore, 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. Therefore, because sensible natures are able not to exist or because they are not always joined to existence, because they begin to exist sometime, we can say that they are in potency to existence and that they have no essence really distinct from existence."
There we have it, Giles of Romes' famous or, if your name is Cunningham, infamous separability criterion added to the real distinction. This set of theorems was Giles' first examination of the topic, save perhaps for his Sentences. Henry reacted to this treatise, which prompted a series of quodlibetal debates between the two. Giles eventually wrote a series of disputed questions about essence and existence, attacking Henry and clarifying his own position. This debate seems to have been heavily influential for the way in which later scholastics interpreted the formal distinction, although the only real literature on this is from thomist scholars, who have notorious blindspots. The separability criterion was certainly incorporated by Scotus and his early followers, and probably became standard among other schools as well. The first question of Peter Thomae's QQ. de modis distinctionis is about it, for example. Comparing this with Thomas of Sutton's version of the real distinction, which is obtains between anything distinct prior to the operation of the intellect, one could note that neither are explicitly found in the writings of Aquinas himself, but have been filled in by his later students (which has provided fodder for plenty of modern thomists, who, forgetting that thomism was not always as dominant as it was from the period 1879-1965, generally attack both for their alleged weak adherence to thomistic doctrine). It would be an interesting study to see if other theologians advanced similar views in the 1280's and on, and to see if the version of Thomas of Sutton might be the Oxford real distinction while that of Giles the Parisian real distinction. But that I will leave to the Thomists.
Here is another quote, from the introduction, p.ix:
"A clear thread runs from Duns Scotus, through William of Ockham to Suarez in the explanation of the distinction between essence and existence. There has been, on the part of these men, a violent reaction to the real distinction between essence and existence, as proposed by St. Thomas. This opposition is based on their rejection of essence and existence as real physical entities which, so they thought, would be required in the real distinction of St. Thomas. In other word, for these men the real distinction is only valid if applied to physical entities which are separable. Such, of course, is not the doctrine of St. Thomas; nevertheless these men have rejected the real distinction for this reason. Why historically this particular doctrine of physical entities was involved in the Thomistic real distinction is not, at the present time, so clear as historians of philosophy would like. It does seem, however, that Giles of Rome is at least partly responsible for this identification of the real distinction with physical entities."
