We now come to one of the most interesting parts of Sun Tzu's treatise, the discussion of the CHENG and the CH`I." As it is by no means easy to grasp the full significance of these two terms, or to render them consistently by good English equivalents; it may be as well to tabulate some of the commentators' remarks on the subject before proceeding further. Li Ch`uan: "Facing the enemy is CHENG, making lateral diversion is CH`I. Chia Lin: "In presence of the enemy, your troops should be arrayed in normal fashion, but in order to secure victory abnormal maneuvers must be employed." Mei Yao-ch`en: "CH`I is active, CHENG is passive; passivity means waiting for an opportunity, activity beings the victory itself." Ho Shih: "We must cause the enemy to regard our straightforward attack as one that is secretly designed, and vice versa; thus CHENG may also be CH`I, and CH`I may also be CHENG." He instances the famous exploit of Han Hsin, who when marching ostensibly against Lin-chin (now Chao-i in Shensi), suddenly threw a large force across the Yellow River in wooden tubs, utterly disconcerting his opponent. [Ch`ien Han Shu, ch. 3.] Here, we are told, the march on Lin-chin was CHENG, and the surprise maneuver was CH`I." Chang Yu gives the following summary of opinions on the words: "Military writers do not agree with regard to the meaning of CH`I and CHENG. Wei Liao Tzu [4th cent. B.C.] says: 'Direct warfare favors frontal attacks, indirect warfare attacks from the rear.' Ts`ao Kung says: 'Going straight out to join battle is a direct operation; appearing on the enemy's rear is an indirect maneuver.' Li Wei-kung [6th and 7th cent. A.D.] says: 'In war, to march straight ahead is CHENG; turning movements, on the other hand, are CH`I.' These writers simply regard CHENG as CHENG, and CH`I as CH`I; they do not note that the two are mutually interchangeable and run into each other like the two sides of a circle [see infra, ss. 11]. A comment on the T`ang Emperor T`ai Tsung goes to the root of the matter: 'A CH`I maneuver may be CHENG, if we make the enemy look upon it as CHENG; then our real attack will be CH`I, and vice versa. The whole secret lies in confusing the enemy, so that he cannot fathom our real intent.'" To put it perhaps a little more clearly: any attack or other operation is CHENG, on which the enemy has had his attention fixed; whereas that is CH`I," which takes him by surprise or comes from an unexpected quarter. If the enemy perceives a movement which is meant to be CH`I," it immediately becomes CHENG."