所不守 is of course hyperbolical; Wang Hsi rightly explains it as “weak points; that is to say, where the general is lacking in capacity, or the soldiers in spirit; where the walls are not strong enough, or the precautions not strict enough; where relief comes too late, or provisions are too scanty, or the defenders are variance amongst themselves.”
You can ensure the safety of your defence if you only hold positions that cannot be attacked.
I.e., where there are none of the weak points mentioned above. There is rather a nice point involved in the interpretation of this latter clause. Tu Mu, Ch‘ên Hao, and Mei Yao-ch‘ên assume the meaning to be: “In order to make your defence quite safe, you must defend even those places that are not likely to be attacked;” and Tu Mu adds: “How much more, then, those that will be attacked.” Taken thus, however, the clause balances less well with the preceding—always a consideration in the highly antithetical style which is natural to the Chinese. Chang Yü, therefore, seems to come nearer the mark in saying: “He who is skilled in attack flashes forth from the topmost heights of heaven [see [IV. § 7]], making it impossible for the enemy to guard against him. This being so, the places that I shall attack are precisely those that the enemy cannot defend.... He who is skilled in defence hides in the most secret recesses of the earth, making it impossible for the enemy to estimate his whereabouts. This being so, the places that I shall hold are precisely those that the enemy cannot attack.”
8. 故善攻者敵不知其所守善守者敵不知其所攻
Hence that general is skilful in attack whose opponent does not know what to defend; and he is skilful in defence whose opponent does not know what to attack.
An aphorism which puts the whole art of war into a nutshell.
9. 微乎微乎至於無形神乎神乎至於無聲故能爲敵之司命
O divine art of subtlety and secrecy! Through you we learn to be invisible, through you inaudible;
Literally, “without form or sound,” but it is said of course with reference to the enemy. Chang Yü, whom I follow, draws no sharp distinction between 微 and 神, but Tu Mu and others think that 微 indicates the secrecy to be observed on the defensive, and 神 the rapidity to be displayed in attack. The Yü Lan text differs considerably from ours, reading: 微乎微乎故能隱於常形神乎神乎故能爲敵司命.
and hence we can hold the enemy’s fate in our hands.