Wow, a nice one. For me, there rises a philosophical question. How could anyone claim to have complete knowledge of (any) topic, if we can never be certain that we know everything there is about somehing. The biggest issue I see is that when we realize that we don't know everything, we also realize there is no way to know how much or what percentage of everything we don't know.

So in any case, the only thing we can be certain of is that we never know how (in)complete our knowledge is.

Thank you for provoking this thought!