The entrepreneurs dilemma: on the right path or delusional?

All entrepreneurs face the moment(s) when they must assess the feedback from customers, the market, investors, employees and people in general as to whether their idea is worthy of becoming a profitable self-sustaining business or weather it's just a futile fools errand.  This question comes up more frequently in the early stages of a business as entrepreneurs go through the difficult time of getting a company off the ground. My only comment to entrepreneurs in this dilemma is the advice that the martial arts instructor gave to us regarding our young son's desire to earn a black belt:
"Every kid is capable of earning a black belt. It's usually the parents who get tired and stop encouraging the goal of obtaining a black belt."   It sounded better when he said it-- my point is that at these dilemma points, perseverance is more frequently than not the answer.