Sun Guoting (648?-703?) stated in his Manual on Calligraphy that calligraphy reveals the character and expresses the emotions of the writer. Few works demonstrate this principle as clearly as this handscroll by Mi Fu, the leading late Northern Song calligrapher. Mi wrote Poem Written on the Wu River with a suspended arm, working from the elbow rather than the wrist. The brush moves lightly, changing pressure within each stroke; the size of the individual characters, the thickness of the brushstrokes, and the amount of ink used vary dramatically from column to column. His aim was not to form perfect individual characters; instead he entrusted his writing to the force of the brush-giving full reign to idiosyncratic movements, collapsing and distorting the forms of the characters for the sake of expressiveness. Su Shi (1036-1101) described Mi's writing style as a 'sailboat in a gust of wind or a war-horse charging into battle.'