Ts‘ao Kung sums up very well: 出空擊虛避其所守擊其不意 “Emerge from the void [q.d. like “a bolt from the blue”], strike at vulnerable points, shun places that are defended, attack in unexpected quarters.” The difference of meaning between 空 and 虛 is worth noting.
7. You can be sure of succeeding in your attacks if you only attack places which are undefended.
所不守 is of course hyperbolical; Wang Hsi rightly explains it as “weak points; that is to day, 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. 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 in a nutshell.