Bob suggests that my belief in the possibility of flying by concentrating very, very hard, is not reasonable because
there is strong evidence against the claim of flying
but while I admit that there is no evidence either for or against levitation there is historical evidence of a very prominent sect back in the 1980s who believed that they could indeed levitate. In fact they were persecuted for their beliefs because the establishment of the day found their beliefs threatening. The ridicule that they endured inspires me to concentrate even harder.
Sure, the ability to fly by concentrating very, very hard is quite unlikely and would violate all the known laws of the natural universe but my belief in it contributes to my happiness which, in turn, justifies my belief.