Macpo

"It is well known that some chess players are able to play several games at the same time, without looking at the boards. [...] And, thus, playing "blindly", mentally representing at any moment the position of each piece on each board, they manage to win, often against skilled players, the several simultaneous games. [...]

There would be here, according to [Taine], a purely visual memory. The player would see, as in an inner mirror, the image of all his boards with all their pieces, as it appears at the last played move. [But] the image of the board is not presented as such to the memory of the player, as "in a mirror"; it requires, at any moment, an effort from the player to reconstitute the situation. What is this effort? [...] Chess players first agreed to declare that the mental vision of pieces would be more annoying than useful. What they remember and represent to themselves of every piece, is not its outer aspect, but its power, its range and value, finally its function. A bishop is not a piece of wood with a more or less strange form; it is an "oblique force". A rook is a certain capacity to walk "straight" [...]. What is present to the memory of the player is a certain composition of forces, even a certain relation between allied and enemy forces."

Bergson, The Intellectual Effort.