Mystici Corporis Christi §62

⁶² But if we compare a mystical body with a moral body, it is to be noted that the difference between them is not slight; rather it is very considerable and very important. In the moral body the principle of union is nothing else than the common end, and the common cooperation of all under the authority of society for the attainment of that end; whereas in the Mystical Body of which We are speaking, this collaboration is supplemented by another internal principle, which exists effectively in the whole and in each of its parts, and whose excellence is such that of itself it is vastly superior to whatever bonds of union may be found in a physical or moral body. As We said above, this is something not of the natural but of the supernatural order; rather it is something in itself infinite, uncreated: the Spirit of God, who, as the Angelic Doctor says, "numerically one and the same, fills and unifies the whole Church."