Ask HN: Is it common for founders to doubt their ideas early on?
... and still go with it? Is it a bad sign if you think "this is a stupid idea, no one will buy it" before you even begin? I don't mean thinking it constantly, but from time to time.
Phase 1. Idea Strikes. Absolutely no doubts. The idea will kick ass and change the world.
Phase 2. Idea development, customer discovery. Doubt creeps in and starts to spread its tentacles. Now the numbers and figures start to play a role in the strength of the doubt. More optimistic figures weaken the doubt and fewer "likable" numbers strengthen the doubt.
Phase 3. The fight goes on. As orangethirty says: "Everyday".
His key point is identifying potential customers and a way to attract them to your product. If you can't identify those 2 things from the start your idea/invention may not be worth pursuing further.
Recommended read on this topic: http://pandodaily.com/2013/02/05/living-with-doubt/