The argument is this. -- The Church is one society, as it is one Church, cf. Ephesians iv, 4-6: v, 23-31: John x, 16). But there is one society only where there is one government; and one government means one sovereign authority (this is an axiom of political science). Is that authority vested in many persons collectively or in one person? We may suppose that the government of the whole Church is analogous to the government of those local Churches, which are called dioceses. But a diocese is governed by one person, the Bishop: therefore the whole Church by one Bishop, supreme over other Bishops. For the value of the argument, cf. note [4.76a]. The conclusion is finally evinced a posteriori by documents and facts of history.

Of God and His Creatures: 4.76