In order to carry out an attack with fire, we must have means available.
Ts‘ao Kung thinks that 姦人 “traitors in the enemy’s camp” are
referred to. He thus takes 因 as the efficient cause only. But
Ch‘ên Hao is more likely to be right in saying: 須得其便不獨姦人 “We
must have favourable circumstances in general, not merely traitors
to help us.” Chia Lin says: 因風燥 “We must avail ourselves of wind
and dry weather.”
The material for raising fire should always be kept in readiness.
煙火 is explained by Ts‘ao Kung as 燒具 “appliances for making
fire.” Tu Mu suggests 艾蒿荻葦薪芻膏油之屬 “dry vegetable matter,
reeds, brushwood, straw, grease, oil, etc.” Here we have the
material cause. Chang Yü says: 𫎓火之器燃火之物 “vessels for
hoarding fire, stuff for lighting fires.”