 In the Middle Ages, we see an important development in theology and that's where the term transubstantiation does become explicit. Because there were certain people, Beringer of Tours is one of the greatest examples of this, of people saying, well, you know, the Eucharist is just symbolic. And so the church had to say, no, there is a substantial presence. What does that mean? Well, substance is what a thing truly is. And so to say that the Eucharist is not just a symbol of the Lord, it is His true presence. His body and blood are substantially made present within the Eucharist. And Thomas Aquinas becomes the great theologian that lays out even how that happens, right through the sacramental grace of the church.