The principal difficulty against this view is a theological difficulty. As formulated by Urraburu,(295) it appears to involve an ambiguity in the expression “substantial union”. It is briefly this: If the subsistence proper to a complete individual nature adds no positive perfection to the latter, so that the latter necessarily subsists and is a person unless it is actually assumed into a higher personality, and by the very fact that it is not actually so assumed, then the human nature of Christ “is as complete in every way and in every line of substantial perfection, by virtue of its own proper entity, when actually united with the Divine Person, as it would be were it not so united, or as the person of Peter, or Paul, or any other human person is”. But this implies that there are in Christ “two substances complete in every respect”. Now between two such substances “there cannot be a substantial union,” a union which would constitute “one being,” “unum per se ens”. Hence the view in question would appear to be inadmissible. But it is not proved that the union of “two substances complete in every respect” cannot result in the constitution of a being that is really and genuinely one—“unum per se ens”—_in the case in which the union is a personal union_. The hypostatic union of the human nature of Christ with the Divine Person is primarily a _personal_ union whereby the former nature subsists by and in the Divine Personality. It has the effect of constituting the united terms “one subsisting being,” and therefore has supereminently, if not formally, the effect of a “substantial union”. Nay, it is a “substantial” union in the sense that it is a union of two substances, not of a substance and accidents; and also in the sense that it is not a mere accidental aggregation or artificial juxtaposition of substances, resulting merely in the constitution of collective or artificial unity, a _unum per accidens_. But is it a “substantial” union in the sense that it is such a union of substances as results in one “nature”? Most certainly not; for this was the heresy of the Monophysites: that in Christ there is only one nature resulting from the union of the human nature with the Divine. If then, with Urraburu, we mean by “nature” simply “substance regarded as a principle of action” (71), and if, furthermore, the hypostatic union does not result in one “nature,” neither does it result in one “substance,” nor can it be a “substantial” or “natural” union in this sense.(296) He does not say, of course, that the hypostatic union is a “substantial union” which results in “one nature,” or even explicitly that it results in “one substance,” but he says that the two substances are “substantially conjoined,” “substantialiter conjunguntur”; and he