Many baseball coaches will say that the thinking or the understanding part of the game is the most vital to success for a player. This part of the game is sometimes called the mental pillar and is one of the primary pillars that support the structure of the athlete within the game of baseball (and any sport - these being mental, physical, technical and tactical).
In general terms, a baseball player will commit two types of errors, but one more than the other can have long term effects - mental and physical. Coaches have no problem for the most part accepting physical mistakes; players will clank ground balls, butcher fly balls and look like a bag of hammers searching for a tool box as a hitter. Players will have bad days, where physically all that happens. No problem, we are all human, all make mistakes. In baseball, it is the mental error that has harsh effects both short and long term on players and teams and will often manifest themselves as physical errors (i.e. what looks like a physical error was actually born a few seconds earlier as a mental one).