"Calculation of variations is a more difficult skill to possess. You need to be able to visualize the position in your head, and to calculate what will happen if a certain sequence of moves occurs. You need to keep in mind that it is not necessary to calculate everything. Humans are not machines and cannot to simply ‘test’ all of the moves one by one. The trick is to pick out the right line to calculate and to be able to calculate it deep enough so that you can evaluate the final position accurately enough to win the game. There are some methods that can be used to improve your calculating ability. One is solving tactics problems in your head without moving pieces on the board. You can start with easy problems, and move to the more difficult ones after your calculation skills improve. The other method is more appropriate for advanced players. Playing blindfold games with much weaker opponents dramatically helps your calculating ability. It also helps with visualization skills a lot". - mi volsi e considerai