The term Earth is the name of a form, a phenomenal entity; the term Kun refers to that which uses or takes this form.
Wang Bi establishes the critical distinction that Kun names not Earth as physical substance but the operative principle — submissive pliancy — that animates and employs the Earth form in paired counterpoint to Qian.
, The Classic of Changes: A New Translation of the I Ching as Interpreted by Wang Bi, 1994thesis