They say Botvinnik had 'scientific' approach to chess and Fischer's style was crystal-clear.
What does it mean?
Could you please enunciate about the styles of these masters in a more lucid way?
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Botvinnik believed in peer review. He would write up annotations for his games, and publish them hoping for feedback from other players. He also recommended to his students that they annotate their losses, and also look for mistakes in the games they won. These elements are all part of his 'scientific' approach.
Fischer always kept the endgame in mind, preferred bishops over knights, grabbed pawns if he thought he could get away with it. Mikhail Tal would wait until the time control drew near, and then suddenly start to complicate the position, trying to induce his opponent into blundering. Fischer was much more straightforward, playing the board more than the man, not indulging in psychological ploys as much (at the board anyway). After the game was over, his opponent would look at Fischer's moves, and think they looked obvious in retrospect. These are the factors that make his style 'crystal clear.'
I wouldn't read too much into these descriptions. We like to make up adjectives to describe great people all the time - these are not always rooted in any kind of quantifiable fact. It is not likely that you would become as good as Botvinnik by "copying his style", if that even means something more than "play the same openings".
Having said that, I believe that Botvinnik was, in addition to a great chess player, also a great computer scientist. He was one of the pioneers in using computers to analyze chess positions - hence a possible clue as to where the "scientific" angle comes from. I doubt that his chess ability was somehow contingent on this work - I think he would have been a great chess player even if he was an accountant or historian.
As for "crystal-clear", that sounds to me like the typical thing you would say if you are looking to praise a man of great skill without committing yourself to what you are actually saying. I mean, it's not like you can somehow disprove it.