I think this idiom refers to something different: it's not about the "pedagogical" skills -- everybody who wants to teach or to transfer knowledge needs to be a

I think this idiom refers to something different: it's not about the "pedagogical" skills -- everybody who wants to teach or to transfer knowledge needs to be a good "pedagogue" or should have some basic skills. Those who do have much, much more experience than those who never did -- latter take their wisdom just from books and experiences from others. But if you want to be really good you have to do it yourself, you have to fail, you have to make mistakes to learn from them. Those who stay with teaching, never tried, never made mistakes and so on. Look: if you want to get a lawyer who managing your transaction with a VC. Who would you take? A guy you did +50 transactions and know the entire village or somebody who took all his wisdom out of a book, let's say Venture Deals and is blogging like Scoble?