Sonam Tsemo, 1142-1182, also called Lobpon Sonam Tsemo (bsod nams rtse mo) of the Khon family was the second of the Sakya Jetsun Gongma Nga, (sa skya rje btsun gong ma lnga) who were the five founding patriarchs of the Sakya order. He was also the fourth Sakya throne holder, although he served as active head of the monastery for only a few years. His mother was called Machig Odron (ma gcig 'od sgron). Like his father he remained a layman throughout his life, although he never married or had children. He was identified as the reincarnation of the Indian scholar Durgachandra (or Durjayachandra) the master of Drogmi Lotsawa Shakya Yeshe's ('brog mi lotsawa shakya ye shes) teacher Viravajra, in India.
During his childhood, Sonam Tsemo's main teacher was his father Sachen Kunga Nyingpo (sa chen kun dga' snying po), who was the first Sakya patriarch and the third throne holder. His studies with his father focused on esoteric topics, and it is said that he could recite fourteen esoteric scriptures, including the Hevajra and Samvara tantras by the age of sixteen. He received oral Lamdre (lam 'bras) instructions from Sachen during this time. After Sachen passed away, Sonam Tsemo's education was strongly inflected by the Indian monastic model. At seventeen he went to the Kadampa monastery Sangpu Neutog (gsang phu ne'u thog) to study Madhyamaka philosophy and epistemology with the great master Chapa Chokyi Sengge (phya pa chos kyi seng ge). This teacher had disciples from several of the most prominent families in Central Tibet and Sonam Tsemo's biography claims that he became the most accomplished of the students. He studied with this master on and off for eleven years and became well versed in Mahayana texts such as Pramanavinischaya and Bodhicharyavatara. Sonam Tsemo also received some instruction from the Indian or Nepali Acharya Shri Anandagharba.
Sonam Tsemo's work Chola jugpai go (chos la 'jug pa'i sgo), which he composed at the age of twenty-six at Nalatse, was extremely influential on the work of his nephew Sakya Pandita Kunga Gyaltsen (sa skya paN+Di ta kun dga' rgyal mtshan), the fourth Sakya patriarch and a widely renowned scholar. Sonam Tsemo's written works address topics including the Bodhicharyavatara, a schematization of the tantra; an explanation of the last two chapters of the Hevajra root tantra; a commentary on the Samputa tantra; instructions for reading Sanskrit, and commemorative texts for his main teachers.
Sonam Tsemo first gave the Lamdre teachings in Sakya at the age of twenty-eight. Many famous masters attended the teaching, and he became renowned as a clear and skilled teacher, but his biographical data reflect a career more focused on study, practice and composition of texts than on teaching. His few close disciples included his brother Dragpa Gyaltsen (rje btsun grags pa rgyal mtshan), Ngodrup (dngos grub), Chagkyi Dorje (lcags kyi rdo rje), and Tsugtor (gstug tor). He was the active Sakya throne holder for only three years, after which he passed the responsibility on to his younger brother Dragpa Gyaltsen, in order to devote the rest of his life to study and retreat.
Sonam Tsemo passed away in 1182 at the age of forty. The details of his death are unclear, but it is recorded that his body disappeared and he left nothing but his robe and a footprint behind.
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