PART II

UNITY IN DUALITY, OR THE HYPO- STATIC UNION
We have shown that there are in Christ two natures, a divine and a human. How are these natures united ?
Ordinarily there are two species of unity, t. e., two modes by which separate substances can be united into one. The first, called accidental (unitas acciden- talis), is that by which two substances loosely coexist, as, e. g., wine and water poured into the same cup. The second, called substantial unity {unitas substan- tialis), is that by which two substances combine so as to constitute a third, which is identical with neither of the two components but forms an entirely new substance. Thus man results from the union of body and soul, water from a combination of oxygen and hydrogen. Moral unity (unitas moralis) is a subdivision of acci- dental unity and obtains chiefly between rational beings, e. g., between Christ and the faithful who receive Him in the Blessed Eucharist, betAveen God and the elect endowed with the beatific vision, etc. Opposed to moral is physical, which necessarily involves substantial unity.
Both reason and experience tell us that two finite substances can be combined into a new substance only
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by losing each its own proper self-existence. It is in this manner that soul and body unite in forming man.
The case is different with our Divine Saviour. In Him Divinity and humanity enter into a peculiar kind of physical and substantial union, in which neither loses its substantial existence. The Divine Logos simply possesses both natures without commingling or blending them to- gether — the divine per modum identitatis realis, the human per modum unitionis. This peculiar kind of phys- ical and substantial union, concerning which we have no knowledge other than that derived from Divine Revela- tion, is technically called hypostatic (unitas hypostatica), in contradistinction to a purely natural or a merely acci- dental or moral union.
The exceptional rank which this " unity in duality " holds among the different species of substantial unity leads us to expect that it should be subject to extraor- dinary determinations and productive of peculiar and unique effects. This is indeed the case, as we shall show in explaining (i) the Hypostatic Union as such, and (2) its effects.