Hence we consider the view which regards real unity of nature as compatible with real and actual plurality of complete substances in the individual, as improbable. At the same time we do not believe that this view is a necessary corollary from the real identification of essence with existence in created things. We have seen that even if accidents have their own existence in so far as they have their own essence—as they have if essence and existence be really identical—nevertheless the concrete substance as determined by its accidents can have a really unitary existence, _unum esse_ corresponding to and identical with its composite constitution (67). Similarly, if the existence of each incomplete substance is identical with its incomplete essence, this is no obstacle to the complete substance—which results from the union of two such incomplete substantial principles—having one complete unitary existence identical with its composite essence. Hence it is useless to argue against the view that a plurality of actually distinct and complete substances can unite to form a complete nature which will be really _one being_, on the ground that each complete substance has already its own existence and that things which have and preserve their own existence cannot form _one being_. Such an argument is inconclusive; for although _one being_ has of course only one existence, it has not been proved that this one existence cannot result from the union of many incomplete existences: especially if these existences be identical with the incomplete essences which are admittedly capable of uniting to form one complete essence.
It may, however, be reasonably urged against the opinion under criticism that, since the complete substances are supposed to remain complete and unchanged in their state of combination, it is difficult to see how this combination can be a real union and not merely an extrinsic juxtaposition,—one which remains in reality a merely accidental conjunction, even though we may dignify it with the title of a “natural union”.
And finally it may be pointed out that in this view the operations of the individual have not really _one ultimate_ intrinsic principle at all, since behind the supposed unity of nature there is a more fundamental plurality of actually distinct substances.