The knight of QB3 is under obligation, the moment the enemy gives him the chance, of undertaking an invasion of the center by Kn-Q5.
Aron NimzowitschThe chess world is obligated to organize a match between the champion of the world and the winner of this Carlsbad tournament - indeed, this is a moral obligation. If the world of chess should remain deaf to its obligation, on the other hand, it would amount to an absolutely unforgivable omission, carrying with it a heavy burden of guilt.
Aron Nimzowitsch