Chalice with picture of Christ

The teaching of transubstantiation depends on a distinction proposed by Aristotle between an unchanging essential "substance" and outward appearances or attributes called "accidents." With this distinction in place, medieval theologians could argue that the substance of the bread and wine in the Lord's Supper could be changed into the substance of the body of Christ while retaining the outward appearance or accidents of bread and wine. The doctrine of transubstantiation was affirmed at the Lateran Council IV (1215) and classically formulated later by Thomas Aquinas. In the face of Protestant criticism, it was reaffirmed at the Council of Trent (1545-1563).