Whatever things (rules) are proposed to you [for the conduct of life] abide by them, as if they were laws, as if you would be guilty of impiety if you transgressed any of them. And whatever any man shall say about you, do not attend to it: for this is no affair of yours.
Once you have decided that something is good, stick to it as though it were a law. Of couse this is a standard to aim for, and one that is very difficult to attain.
This will be restated (I think more poetically) in part 51.