When it comes to developing ability at something, a craft like this, or perhaps at a sport, anything really, we commonly hear people talking about such things as “it takes ten thousand hours” as though it were simply a matter of spending enough time and making enough repetitions. But that’s not true. […]. It’s no good spending ten thousand hours on practice if you’re going to simply spend the same hour ten thousand times.
You have to be constantly probing and investigating everything you do. Why did it turn out this way? What was wrong? What do I have to do to make it better next time? Each and every one of those ten thousand hours or whatever the magic number might be has to be spent in such analysis. Mindless repetition, no matter how long, will get you nowhere.
—David Bull, ukiyo-e woodblock printer, in a video interview by Andrés Rodriguez Cortés