… is from page 576 of the 1988 collection of Lord Acton’s writings (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; (I can find no date for this passage):
Behind the man of action, and above him, is the thinker. You must keep to the line where they meet – the history of political ideas.
