Tu Mu , after Tsʽao Kung the most important commentator on Sun Tzǔ , composed the preface to his edition 38 about the middle of the ninth century. After a somewhat lengthy defence of the military art, 39 he comes at last to Sun Tzǔ himself, and makes one or two very startling assertions:⁠—“The writings of Sun Wu ,” he says, “originally comprised several hundred thousand words, but Tsʽao Tsʽao , the Emperor Wei , pruned away all redundancies and wrote out the essence of the whole, so as to form a single book in 13 chapters.” 40