… is from page 543 of the 1985 collection of Lord Acton’s writings (edited by J. Rufus Fears), Essays in Religion, Politics, and Morality:
The true liberal descries liberty as an end, not a means. To do something for him and others that no other thing can do. Not exchangeable for any amount, however large, of national greatness and glory, of prosperity and wealth, of enlightenment and morality.
