Parmenides argued that a particular being cannot become another particular being. For example air cannot become fire but must first cease to be air as such. The change of air to fire would be in this case a mere replacement of one being for another. Aristotle answered this problem with his distinction between three factors in change: Form, matter, and privation. A ball of clay is not a vase, but it has the potential for being a vase. In this example clay is the matter, vase is the form, and the privation is the clay’s lack of being a vase. According to Aristotle air can be changed into fire because air is never just air. Air is potential fire. Therefore when it is changed into fire it is not annihilated but part of its potential being (fire) is turned into actual being. An acorn is a potential tree. When an acorn is changed into a tree it does not lose part of its being but has its capacity for treeness fulfilled. This does not mean that change is illusory but that change is real.
This distinction is helpful for understanding Thomas Aquinas on the Eucharist. Although he notes that natural changes are changes of accidental form and not substantial form in the Eucharist the bread and the wine do not lose their composition of essence and existence but are changed from a potential glorified being to actual glorified substances. They in turn actualize other potentialities within the believer – faith resulting in union with Christ through the Holy Spirit. As others have pointed out this change is meta-substantial. It transcends substance to the nature of created being itself. Believers do not lose their substance but receive that substance back fully renewed in Christ. They receive Christ and all of his benefits: his mind, will, nature, etc. Of course, this assumes an ontological, not merely legal, sinful nature in man. Since the Fall man lacks full being but also actualized being of a certain type. Adam was created in God’s image but was intended for glorification in union with God. Just as the acorn was intended to be a tree man’s final cause is a shared being, a mutual indwelling with our incarnate glorified Lord. “And we all, with unveiled face, beholding the glory of the Lord, are being transformed into the same image from one degree of glory to another. For this comes from the Lord who is the Spirit.” [2 Cor. 3:18] Christ’s body on earth is being transformed from a potential body to a pure, glorified, actualized body.