I love reading Seth Godin. From one of his blog posts

If you are unable to declare, then you're going to slog, and instead of starting new projects based on what you've learned, you'll merely end up trapped. I'm not suggesting that you flit. A project might last a decade or a generation, but if it is to be a project, it must have an end.

While you may be tempted to be situational about this, to know it when you see it, to decide as you go, it's far more powerful and effective to define victory or failure in advance.

Jim Carrey put a $10M check signed to himself in pocket even before he got his first big break. Though, it is an exterme example, a goal that marks victory or failure is a great way to keep us in check. Code, career moves, dreams, manuscripts,...all need a firm goal, which is tangible.