The final book of The Imitation of Christ is an ‘exhortation to Holy Communion’, and the last chapter begins: ‘Beware of a persistent and useless intellectual examination of this most deep sacrament, if you do not wish to be immersed in deep doubt.’ Useless, I am afraid, to warn philosophers. In what follows I distinguish two or three different ways of understanding the Real Presence in the Eucharist and, without rejecting any of them, say how far they are compatible and indicate some of their merits and drawbacks. But since this doctrine is sometimes confused with the doctrine of Transsubstantiation, I begin with a word about that.

In 1551 the Council of Trent declared that ‘through the consecration of the bread and wine, there comes about a conversion of the whole substance of the bread into the body of Christ our Lord, and of the whole substance of the wine into his blood. And this conversion is appropriately and properly called “transsubstantiation'” (Denzinger/Schönmetzer 1642). The doctrine of transsubstantiation here endorsed is relatively clear. It states that when bread and wine are changed into the body and blood of Christ, the matter of the bread and wine pass away into the matter of the body and blood of Christ. The purpose of the doctrine is to distinguish the Eucharistic change from natural changes such as occur when water evaporates into air or when bread and wine are consumed in the ordinary way. The Fathers of Trent and the mediaeval philosophers from whom they took the doctrine believed that these natural changes are, strictly speaking, only transformations, conversiones formales (Aquinas, ST 3.75.4).