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Throughout most of Chinese history, the Yellow Emperor and the other ancient sages were considered to be historical figures. Their historicity started to be questioned in the 1920s by historians such as Gu Jiegang, one of the founders of the Doubting Antiquity School in China. In their attempts to prove that the earliest figures of Chinese history were mythological, Gu and his followers argued that these ancient sages were originally gods who were later depicted as humans by the rationalist intellectuals of the Warring States period. Yang Kuan, a member of the same current of historiography, noted that only in the Warring States period had the Yellow Emperor started to be described as the first ruler of China. Yang thus argued that Huangdi was a later transformation of Shangdi, the supreme god of the Shang dynasty's pantheon.