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 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.”