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B.C.] says: 'Direct warfare favours 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 manoeuver.' 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, § 11]. A comment on

the T'ang Emperor T'ai Tsung goes to the root of the matter: 'A

ch'i manoeuver may be cheng, if we make the enemy look upon it as

cheng; then our real attack will be ch'i, and vice versa.