REFERENCE BOOKS. GLOSSARY. MINIEDITIONS
"The sound human mind tells us that Pieces are useless, while they are inactive. And Pieces are inactive till they take part in the attack or defense (assaulting the hostile Piece, pinning, blocking them, supporting one's own Pieces etc.), or, at least, while they are not mobile. Hence the first commandment of a chessplayer follows: to those Pieces which, in the original position, obtain very narrow field of activities, little mobility, and even block their own Pieces, more freedom and wider range of activities should be created..."
Emanuel Lasker, Champion of the World