It is a well known phenomenon that the same amateur who can conduct the middle game quite creditably, is usually perfectly helpless in the end game. One of the principal requisites of good chess is the ability to treat both the middle and end game equally well.
Aron NimzowitschIf in a battle, I seize a bit of debatable land with a handful of soldiers, without having done anything to prevent an enemy bombardment of the position, would it ever occur to me to speak of a conquest of the terrain in question? Obviously not. Then why should I do so in chess?
Aron NimzowitschGiving up the center must not here be regarded as illogical. Was happiness no happiness because it endured for just a short time? One cannot always be happy.
Aron Nimzowitsch