Spies are a most important element in war, because on them depends an army’s ability to move.
The antecedent to 此 must be either 間者 or 用間者 understood from the whole sentence. Chia Lin says that an army without spies is like a man without ears or eyes.
CHINESE CONCORDANCE
Ai 愛 VIII. 12; X. 25, 26; XI. 18, 66; XIII. 2.
„ 阨 X. 21.
an 安 II. 20; V. 22; VI. 4; XII. 22.
Cha 詐 VII. 15.
ch‘a 察 I. 2; VIII. 14; IX. 39; X. 13, 20; XI. 41.
chan 戰 passim.
„ 霑 XI. 28.
chang 障 IX. 21.
„ 仗 IX. 29.
ch‘ang 常 VI. 32, 34; X. 18; XI. 29.*
„ 長 VI. 34.
„ 嘗 V. 9.
chao 朝 VII. 28.
chê 者 passim.
„ 折 V. 13; XI. 63.
chên 軫 XII. 4.*
ch‘ên 陳 VII. 32; IX. 25, 27; X. 18.
„ 塵 IX. 23.
chêng 爭 III. 7; VII. 3, 5, 6, 7, 9, 10, 22: VIII. 3; XI. 1, 4, 11, 47, 55; XIII. 2.
chêng 正 V. 3, 5, 10, 11; VII. 32; XI. 35.
„ 政 III. 3, 14; IV. 16; VII. 23; XI. 32, 56, 63.
„ 整 XI. 18.
ch‘êng 成 III. 4; XI. 62; XIII. 4, 27.
„ 城 II. 2; III. 3, 4, 5, 6; VIII. 3; XI. 7, 55; XIII. 20.
„ 乘^2 II. 4, 17; XI. 19.
Sun Tzŭ on the Art of War: the Oldest Military Treatise in the World · The Wunder Library — complete classics, free to read, with narration.