See Ethics and Natural Law, p. 8, n. 4; and p. 76, n. 4. When Milton says in the Comus
Virtue alone is happiness below,
he cannot reasonably mean that moral virtue is formally and precisely happiness, but only that it is indispensable to happiness, and presupposed, as the base of a tower is presupposed to the spire. Moral virtue is more indispensable, but happiness is better. But the privation of happiness is a less evil than the privation of moral virtue. So it is less evil to have the spire blown down than to have the tower on which it rests blown up, although the spire is higher and nobler than the substructure.
The doctrine of this chapter is in Aristotle, Nic. Eth. X, viii.
Of God and His Creatures: 3.34