FWIW, anyone who's interested can find Mill's book here and the quote in context here.
Time hallows these things, though. In 1862, Mill might have been seen as the Michael Kinsley or Bill Kristol of the day, the fire-breathing non-combattant. Today we look at him differently. The words are true enough, but it's the fact that one of the greats seems to be talking to us directly across a century and a half that gives them their resonance.