I’ve come to believe that this is a universal principle: disbelief regarding feasibility prevents actualization of vision; once one sees that it is done it is as if a mental barrier has been removed.
However, even after the mental barrier is removed - Majority of us do not have the motivation to actualize our visions even after we know and believe that they are feasible.
I suspect that it takes a special human emotional/energy makeup to overcome enormous obstacles in pursuit of actualizing a vision. Thats what makes entrepreneurs/hustlers special.
It just comes down to being able to give it a try, mainly from having a combination of low failure cost and high success probability.
Much of the time the decision not to pursue the vision is the right one, because so much sacrifice is required. A lot of successful entrepreneurs have tragic personal lives because in the pursuit of their vision they sacrifice their families. For those of us who are perhaps older, with a lot of personal commitments, and perhaps are also economically comfortable, those sacrifices are too great.
Person saying it cannot be done, should not interrupt person doing it.
In other words: believe what you will, as long as you don't get in the way of someone who's busy doing what you think is impossible.