Heaven is firm, earth is flexible. Heaven is strength; stronger than strong, it is the ultimate of firmness… Heaven as the Tao is the unceasing circulation of one energy, above and below, eternally so.
This passage establishes Heaven and Earth as the primal complementary pair of the Taoist system—firmness and flexibility, eternal circulation and receptive form—whose conjunction is the foundational cosmological and practical teaching of the I Ching.
, The Taoist I Ching, 1986thesis