It is only the enlightened ruler and the wise general who will use the highest intelligence of the army for the purposes of spying, and thereby they achieve great results.
An army may be likened to water, for just as flowing water avoids the heights and hastens to the lowlands, so an army avoids strength and strikes weakness.
When you know both yourself as well as your competition, you are never in danger. To know yourself and not others, gives you half a chance of winning. Knowing neither yourself or your competition puts you in a position to lose.
You can ensure the success of your attacks if you only attack places that are undefended. You can ensure the safety of your defense if you only hold positions that cannot be attacked. Therefore, that general is skillful in attack whose opponent does not know what to defend; and he is skillful in defense whose opponent does not know what to attack.
Hence the saying: If you know the enemy and you know yourself, your victory will not stand in doubt; if you now Heaven and you know Earth, you may make your victory complete.
If our soldiers are not overburdened with money, it is not because they have a distaste for riches; if their lives are not unduly long, it is not because they are disinclined to longevity.
Thus the highest form of generalship is to balk the enemy's plans, the next best is to prevent the junction of the enemy's forces, the next in order is to attack the enemy's army in the field, and the worst policy of all is to besiege walled cities.
If you are situated at a great distance from the enemy, and the strength of the two armies is equal, it is not easy to provoke a battle, and fighting will be to your disadvantage.