He's not talking about quitting just because you can. No, he is talking about that moment when you realize you are just going through the motions, without interest or passion, and only doing it to mark off some irrelevant check-list.
Don’t push yourself to finish when it’s not crucial to your business to do so. If you have a project that you are doing merely to fill the time and find yourself unmotivated, uninspired, or worse uninterested then stop- close the books, get up from your desk and walk away.
Sometimes we forget just how important it is to listen to those little signals we send ourselves.
He is offering professional advice, but the same lesson holds true for me in my training. Action 173 - Stop The Half-Assed Workouts. When I am strength training, I can feel the difference between giving it my all and just pushing weights around. When I am not into it at all, my form gets sloppy, my posture is poor, and I am more likely to hurt myself than to get any real benefit from the workout.
Again, this isn't to say that I should just quit anytime I don't want to lift weights. That's not the message. But on those occasions when I just don't have it in me to give a full-charge, all-in strength training session, I am better off switching to an hour of cardio and coming back to the weights the next day. I'm in this for the long haul, and that means accepting that not every day is going to be aces at the gym.