The reader has been climbing for seven rooms. Each room presupposed an order it did not itself ground. Reality assumed being. Person assumed soul. Good assumed end. Virtue assumed law. Words assumed truth. City assumed common good. Age assumed standard. The last room is not a new topic. It is the name of the order that has been sustaining the climb.
In the older tradition, that name was God — not as a competing being among other beings, but as the source whose existence is what allows other beings to exist at all. To call God "First Cause" is not to put him at the start of a chain of dominoes. It is to name the activity by which everything else has its being moment to moment, the way music has its sound only as long as the player keeps playing.
Not a being among beings, but the source whose existence is what allows other beings to exist at all.
Why source and end are one name
Aquinas says, with characteristic compactness: God is both the alpha and the omega — the principle from which and the end toward which. The two are the same because the order of being is also the order of love. What gives a thing its existence is what completes it; the source from which it came is the only ground large enough to receive its full return.
Augustine put the same point in the first-person voice: "Thou hast made us for Thyself, and our heart is restless until it repose in Thee." Restless until — not because human beings are constitutionally dissatisfied, but because they were made for a particular kind of rest. Smaller rests are possible and good, but they are previews. The full rest is the return to the source.
The realist tradition does not arrive at God as the conclusion of an argument added on at the end. It finds, looking back from the eighth room, that it has been speaking of him all along. The order whose features the previous rooms charted — the being that does not ask permission, the soul whose form is the body, the good that is fulfillment, the truth that is not a perspective — is the same order that, at its source, has a name.
You came in through reality. You leave through the recognition that the room you have always been in is the antechamber of a larger one. The ascent does not end with the eighth room. The eighth room is the door.