(Originally posted on Quora.)
Simply put, it is because the depth of mastery has no end. As simple as the techniques of sushi are, one can go as deep as he wants to master the art. This, I think, goes with any form of art, not just sushi, and it is a common belief among most Japanese: Never-ending improvement in search for perfection. After being a sushi chef for fifteen years, I feel that I just started to understand how to make sushi rice when I mix rice with sushi vinegar. I still feel that I need lots of practice making sashimi. (please refer to my answer: Sushi: Does a sashimi chef really need that much skill?) I think my nigiri is still far from perfect: could use improvement to reach the perfect fish and rice balance with firmness and texture. Traditional seaweed-out rolls (Tekka/Tuna or Kappa/Cucumber) I make look very different from the ones other sushi masters make. When I compare mine with theirs, I say to myself, "I still have long ways to go."
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