… is from page 545 of the 1988 collection of Lord Acton’s writings and notes to himself (edited by the late J. Rufus Fears), Essays in Religion, Politics, and Morality; specifically, it’s a note drawn from Acton’s extensive papers at Cambridge University:
Declaration of Independence and Wealth of Nations appeared in the same year. Both were theories of liberty, emancipating from history.
DBx: What Acton here means – as is evident from the spirit of his work – is that both documents sought to liberate humanity, not from history, but from the arbitrary power that cursed humanity for most of history.