Sima Tan 司馬談 (190-110 BCE) was a famous historian of the Former Han period 前漢 (206 BCE-8 CE). He started writing China's oldest universal history, the Shiji 史記, that was finished by his son Sima Qian 司馬遷.
Sima Tan hailed from Xiayang 夏陽 (modern Hancheng 韓城, Shaanxi) and began his life as a peasant, yet he had access to education and was therefore known for his vast knowledge. He became a disciple of the astronomer Tang Du 唐都 and of Yang He 楊何, an expert in the Confucian Classic Yijing 易經 "Book of Changes". A third teacher of his was Huangzi 黃子, a Daoist master. With such an education he was appointed aide of the Grand Astrologer (taishicheng 太史丞) under Emperor Wu 漢武帝 (r. 141-87 BCE), and finally was appointed Grand Astrologer (taishiling 太史令).
Sima Tan compiled the book Lun liujia yaozhi 論六家要旨, that was an overview of the main teachings of the six prevalent philosophical schools, the Confucians, Mohists, dialecticians, legalists, Yin-Yang thinkers and the Daoists. In this book he stressed that all schools had made important and useful contributions, yet personally, he preferred Daoist teachings. This was in accordance with the general political climate that had prevailed under the Emperors Wen 漢文帝 (r. 180-157 BCE) and Jing 漢景帝 (r. 157-141 BCE).
Sima Tan had access to historiographical sources like the Guoyu 國語, Zhanguoce 戰國策, Shiben 世本 or Chu-Han chunqiu 楚漢春秋 and started compiling a history that made use of all these different sources, including actual sources in the imperial archives for contemporary history.
In 110 BCE, Sima Tan accompanied Emperor Wu during his travel to Mt. Taishan 泰山 where he intended to perform the fengshan 封禪 sacrifices to Heaven and Earth. Sima Tan fell ill and had to stay in Zhounan 周南, where he died. On his deathbed he entrusted his son Sima Qian with the compilation of the Shiji. Sima Qian finished the book, and he is normally mentioned as the author of the Shiji, although Sima Tan had already written substantial parts of the book.