Fools rush in where angels fear to tread.

Those who challenge "conventional wisdom" are often considered fools. However, experience has shown that so-called "conventional wisdom" is often grossly mistaken. What is considered impossible according to conventional wisdom may well be possible, and the ignorance that is attributed to the "fool" actually lies on the side of conventional wisdom.

We owe some of the greatest achievements of humanity to "fools" who were brave enough to go against the wisdom of the day and attempt to do the "impossible." 

It was thought to be impossible to create a heavier-than-air flying machine—until the Wright Brothers proved that it could be done, and, as a result, we now take air travel in airplanes for granted.

Columbus was considered a "fool" for thinking that he could reach India (which was in the East) by sailing West, but he was, ultimately right (even though he did not reach India).

If we adopt the mindset that something is impossible, we will never have any reason to give it a try in the first place, and we will never discover that we were mistaken. But if we adopt the "ignorant" mindset of a "fool," we make no assumptions about what is impossible and what is not. We do not carry around with us the baggage of conventional wisdom. The fool's open-mindedness to all possibilities enables him or her to think in fresh ways, to "think outside the box" (as the popular cliche has it), and therefore to achieve what was though to be impossible.