From Pirates of the Caribbean, Captain Jack Sparrow speaking:
“The only rules that really matter are these: what a man can do and what a man can’t do.”
For myself, I’d have to expand it, though, to say “The only rules that really matter are these: what a man can and will do, and what a man can’t or won’t do.” Because there are lots of things I could do, that I won’t.
-Popgun