Like "A Moral Victory" (Finish the game with no kills or knockouts), this achievement will come naturally if you use the recommended settings for:
The Something to Prove achievement in THIEF worth 554 pointsFinish the game with a custom difficulty of 700 points or more
Or if you just turn on "No Focus" in the custom difficulty screen when you're starting a new game. This setting literally disables focus so that it cannot be used, so you'll get this achievement if you can just finish the game with this setting on.
If you are playing without this setting enabled and accidentally use focus, you should be able to reload your checkpoint as long as it's before your focus use. That's according to reports, I cannot personally verify that.