Tiān (天) is one of the oldest Chinese terms for heaven and a key concept in Chinese mythology, philosophy, and religion. During the Shang Dynasty (17–11th centuries BCE), the Chinese referred to their supreme god as Shàngdì (上帝, "Lord on High") or Dì (帝,"Lord"). During the following Zhou Dynasty, Tiān became synonymous with this figure. Heaven worship was, before the 20th century, an orthodox state religion of China. In Taoism and Confucianism, Tiān is often translated as "Heaven" and is mentioned in relationship to its complementary aspect of Dì (地), which is most often translated as "Earth". These two aspects of Daoist cosmology are representative of the dualistic nature of Taoism. They are thought to maintain the two poles of the Three Realms (三界) of reality, with the middle realm occupied by Humanity (人, Rén).