My critical thinking professor told an anecdote:

“This is a pen; how do we know? Because it does everything that a pen does. And that’s good enough for now, but when the pen gets up and starts talking, it’s time to reevaluate whether it’s still a pen.”

I think that applies here too. We are the sum of our experiences and education is a part of that experience, eventually we have enough knowledge in a particular subject and a concept for gaining new knowledge. So, to a certain extent it never happens, we should always be asking questions.