The great Indian mystic Yogi Berra once said "In theory, there's no difference between theory and practice. In practice, there is." There is also the story about the economist who saw something working in practice and ran hurriedly back to his study to see if it worked in theory.
We have a view of experts that they know a certain amount, and so if we need to make a guess into the unknown, they are staring out from a higher platform, and are thus better prepared to make that guess. Nicholas Nassim Taleb states that his experience is the opposite: experts who guess beyond what they know are actually worse estimators than talented amateurs who have an awareness that they are guessing. His thought is that solid knowledge for any decision can be sparse, and "experts" who study a topic move quickly in to the area of guesswork and speculation. But because these guesses are in consensus and are part of a culture, the experts come to believe that this knowledge is just as solid as the basic data. This spins quickly out of control into intellectual fashions that the experts are agreed on, but are shakier and shakier with each successive floor added to the building.
Taleb suggests a reverse rule. Every yard higher that an expert believes he knows beyond what he actually does know should be subtracted from his real knowledge, as it represents something that will need to be unlearned, which is difficult. Something like "absolute value," if you remember that concept from high school algebra - guessing a furlong beyond one's actual knowledge is equivalent to being a furlong short in actual knowledge. Thus, even a brilliant person with a lot of knowledge can throw it away and become useless by pretending to know what is actually unknown. Thus the virtue of humility and/or the discipline of skepticism become as important as knowledge.
I did mention that "skepticism" has only recently come to be applied primarily to religious questions, didn't I? Until the 20th C (maybe late 19th), it applied to being skeptical about other knowledge and how we arrived at it.