In his "Manual of Chess" Lasker had a theory----at first there were players with more natural tactical ability who were recognized as being the "stronger" players. Then some unknown player started writing down the games of these stronger players and showing them to his friends. The result was that weaker players starting beating stronger players merely by following their own moves.
So it follows that players got better by using ideas from "coaches" or whatever you want to call them.
I read Silman's article (http://www.chess.com/article/view/how-to-turbo-charge-your-game) where he talks about using a chess coach to improve faster than going it alone. Books also help us to improve faster than we could on our own.
So in those examples, improvement happens because someone who knows more than we do tells us something we don't know. But there must be another way!
There must be another way, otherwise the first chess player could never have improved. The first chess player obviously did not have a coach, or better players to explain things to him, or books to learn from, or master games to study. So how did he improve?
Obviously the first chess player was terrible, so I'm not specifically interested in how that person improved. I'm interested in general, how someone would improve on their own without any outside resources?