text: No treatise.  But it's ultimately from Jesus that we get the notion of transubstantiation to begin with.  It needed some "'splaining to do, Lucie" by theologians, but theologians who know Christ's words have taught for centuries that his Real Presence in the Eucharist is different from his presence when two or three are gathered in his name.  I wish I could steer you to theology on the point, but I don't have any handy.  Google it.  You'll find it.