I think what he meant is that it's impossible to "make" someone a great programmer, as in "transform him by applying external force". If so, then I agree - it's impossible to "teach" or "convince" someone to write for example readable code, one must learn on his own through practice.
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Don't see why one would call it a bad question.
A bad question is one to which everyone knows the answer to (or everyone knows that no one knows the answer to).
"It's absolutely impossible to turn an average coder into a great coder." - Sorry all great coders were average at some point. No one is born great.
I think what he meant is that it's impossible to "make" someone a great programmer, as in "transform him by applying external force". If so, then I agree - it's impossible to "teach" or "convince" someone to write for example readable code, one must learn on his own through practice.
I think you can turn an average coder into a good one. Just not a average / good one into a great one.