When I teach project management, I teach people to know what success means, and to know what done means (release criteria). One of my students recently emailed me:
At work recently, we've come upon a scenario where we have no success criteria (or more accurately, success criteria that we can measure in any way). We do have fairly clear failure criteria. Our only measurable success criterion will be a continuing absence of failure.
Well, that's a project that has little chance of ending.
Knowing you haven't failed is not the same as succeeding. I use success criteria to manage the tradeoffs during a project and release criteria to evaluate progress towards done-ness, so we can get to a successful project.