16. While heeding the profit of my counsel, avail yourself also of any helpful circumstances over and beyond the ordinary rules. 17. According as circumstances are favourable, one should modify one’s plans. [Sun Tzŭ, as a practical soldier, will have none of the "bookish theoric." He cautions us here not to pin our faith to abstract principles; "for," as Chang Yu puts it, "while the main laws of strategy can be stated clearly enough for the benefit of all and sundry, you must be guided by the actions of the enemy in attempting to secure a favourable position in actual warfare." On the eve of the battle of Waterloo, Lord Uxbridge, commanding the cavalry, went to the Duke of Wellington in order to learn what his plans and calculations were for the morrow, because, as he explained, he might suddenly find himself Commander-in-chief and would be unable to frame new plans in a critical moment. The Duke listened quietly and then said: "Who will attack the first tomorrow—I or Bonaparte?" "Bonaparte," replied Lord Uxbridge. "Well," continued the Duke, "Bonaparte has not given me any idea of his projects; and as my plans will depend upon his, how can you expect me to tell you what mine are?" [1] ] 18. All warfare is based on deception. [The truth of this pithy and profound saying will be admitted by every soldier. Col. Henderson tells us that Wellington, great in so many military qualities, was especially distinguished by "the extraordinary skill with which he concealed his movements and deceived both friend and foe."] 19. Hence, when able to attack, we must seem unable; when using our forces, we must seem inactive; when we are near, we must make the enemy believe we are far away; when far away, we must make him believe we are near. 20. Hold out baits to entice the enemy. Feign disorder, and crush him. [All commentators, except Chang Yu, say, "When he is in disorder, crush him." It is more natural to suppose that Sun Tzŭ is still illustrating the uses of deception in war.] 21. If he is secure at all points, be prepared for him. If he is in superior strength, evade him. 22. If your opponent is of choleric temper, seek to irritate him. Pretend to be weak, that he may grow arrogant. [Wang Tzŭ, quoted by Tu Yu, says that the good tactician plays with his adversary as a cat plays with a mouse, first feigning weakness and immobility, and then suddenly pouncing upon him.] 23. If he is taking his ease, give him no rest. [This is probably the meaning though Mei Yao-ch’en has the note: "while we are taking our ease, wait for the enemy to tire himself out." The _Yu Lan_ has "Lure him on and tire him out."] If his forces are united, separate them. [Less plausible is the interpretation favoured by most of the commentators: "If sovereign and subject are in accord, put division between them."] 24. Attack him where he is unprepared, appear where you are not expected. 25. These military devices, leading to victory, must not be divulged beforehand.