Be ambitious enough to start and honest enough to stop if it’s clearly not working.

Two different virtues are needed at two different moments, and most of us only have one. Starting takes the nerve to look foolish; stopping takes the nerve to admit in public that you were wrong, so people keep going well past the point where the evidence is clear. Persistence is not automatically a good thing. It is good when something works, or when you have real reason to think it will.

Decide now what evidence would tell you to stop, and write it down while your head is clear. Then give the thing a fair run before you look. Quitting early and never quitting both dodge the question.