Appendix II. Excursus on the Meaning of Concomitance
This kind of necessary presence of soul wherever a living physical human body happens to be located is technically called presence by concomitance. For example, suppose a man's body is conveyed from one place to another by a railway train. His soul is not moved by the train, because his soul is an immaterial element of human nature. The physical train moves the man's physical body. But when this hypothetical man alights at his destination, his soul (as well as every other element of his individual person) is also present. If this were not so the man would not be alive. But the soul is here present at the destination, not because the train moved it, but because of a necessary concomitance with the living man's body. The train could, of course, equally well convey a dead man's body, but at the destination there would arrive no human soul, because there would be in this case no union of soul and body of the dead individual; there would be no presence of the dead man's soul by concomitance with his body.
It is after this fashion that Our Lord, as a human soul, and also as God, is present in the Sacrament of His Body and Blood. In the Sacramental Elements He has provided that the Substances of His risen human Body and Blood shall emerge within our natural world. His own complete and total Presence follows necessarily, because His human soul is united to His human Body, while these are both also inseparably united in living unity with the wholeness of His Divine Person. Our Lord is present in the wholeness of His two natures by a necessary concomitance with the Sacramental Presence of the Substances of His Body and His Blood. Cf. St. Thomas Aquinas, Summa Theologica: III, Q LXXVI, Art. 1. Saint Thomas points this up by saying that if the Sacrament had been consecrated during the three day period after the Cross, while Our Lord's Body and Blood were not united to His human soul, nor to His Godhead, then Our Lord's dead Body and Blood alone would have been present. Our Lord would no more have been present on that hypothetical Altar in His wholeness than He was totally present in His tomb while His human body lay there dead.