SUN ZI said:  The skilful warriors in ancient times first made themselves invincible and then awaited the enemy's moment of vulner ability. Invincibility depends on oneself, but the enemy's vulnerability on himself.  It follows that those skilled in war can make themselves invincible but cannot cause an enemy to be certainly vulnerable.  Therefore, it can be said that, one may know how to achieve victory, but cannot necessarily do so.

    Invincibility lies in the defence;  the possibility of victory in the attack.  Defend yourself when the enemy's strength is abundant; and attack the enemy when it is inadequate.  Those who are skilled in defence hide themselves as under the most secret recesses of earth; those skilled in attack flash forth as from above the topmost heights of heaven.  Thus, they are capable both of protecting themselves and of gaining a complete victory.

    To foresee a victory no better than ordinary people's foresight is not the acme of excellence.  Neither is it the acme of excellence if you win a victory through fierce fighting and the whole empire says, 'Well done!', Hence, by analogy, to lift an autumn hair does not signify g~eat strength; to see the sun and moon does not signify good sight; to hear the thunderclap does not signify acute hearing. In ancient times,  those called skilled in war conquered an enemy easily conquered.  Consequently, a master of war wins victories without showing his brilliant military success, and without gaining the reputation for wisdom or the merit for valour.  He wins his victories without making mistakes.  Making no mistakes is what establishes the certainty of victory, for it means that he conquers an enemy already defeated. Accordingly, a wise commander always ensure that his forces are put in an invincible position, and at the same time will be sure to miss no opportunity to deteat the enemy. It follows that a triumphant army will not fight with the enemy until the victory is assured, while an army destined to defeat will always fight with his opponent first, in the hope that it may win by sheer good luck.  The commander adept in war enhances the moral influence and adheres to the laws and regulations,  Thus it is in his power to control success.

    Now, the elements of the art of war are first, the measurement of space; second, the estimation of quantities; third, the calculation of figures; fourth, comparisons of strength and fifth, chances of victory.  Measurements of space are derived from the ground.  Quantities derive from measurement, figures from quantities, comparisons from figures and victory from comparisons.

    Therefore,  a victorious army is as one y balanced against a grain,  and a defeated army is as a grain balanced against one yi.

    An army superior in strength takes action like the bursting of pent-up waters into a chasm of a thousand fathoms deep. This is what the disposition of military strength means in the actions of war.