Mencius
The culture that Mencius came from believed that after people died they would have some kind of existence after life. That belief goes back to distant prehistorical times. At some time, people began to say that the dead go to 天 Tiān. Tiān has several meanings including "sky," "day," "Heaven," and "God." The earliest versions of the character 天 is a picture of a man with a very large head, which suggests "head man" or "leader." Since Chinese people believed that when any person dies he or she goes to Tiān, and that dead ancestors are more powerful than they were while still alive, it is possible that Tiān originally was the name of the earliest ancestor of the Zhōu ruling house from long before the fall of the previous dynasty, the Shàng. If that idea is correct, then "Tiān" first was a name for an ancestor. Later people may have forgotten that this very powerful spirit was originally a living human being, so Tiān became regarded as a god. After that, "Tiān" could have been used to name the place where the god lived, and since that place was high above us, it came to mean not only "Heaven," but also "the heavens," "sky," "day," etc.