… is from page 648 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):
Every man is the best, the most responsive judge of his own advantage. Therefore don’t let some one else interfere.
