However, the defender should avoid making concessions until forced, and then should make only the minimum concession necessary to meet the threats. The attack is to be directed against the weakest spot in the opposing position. Purdy and Hooper disagree on what constitutes the theory. This of course assumes that the loser must have made at least one mistake , and Speelman quite explicitly signs up to the theory that there is Justice in chess, and that to lose you must have made a mistake - or two, really, and maybe three if you are White. An annotation in Chess Monthly where the GM reluctantly admitted that despite a series of brilliant moves by Black, White was at no disadvantage.
Different aspects have different importances.
Defensive play may be so poor that an attack undertaken with insufficient means might well succeed in practice. So, defensive play, as modelled by Paulsen and Steinitz, must become a strength of yours.