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This term is an established theological term within Hindu philosophy which denotes a specific line of theological development within a Hindu religious tradition. The traditional schools of Hindu philosophy have had their siddhantas established by their respective founders in the form of Sutras (aphorisms). The Sutras are commented by a major philosopher in the respective traditions to elaborate upon the established doctrine by quoting from the shastras (scriptures) and using logic and pramanas (accepted source of knowledge). For example, in the tradition of Vedanta, the author of the Brahma Sutra was Veda Vyasa and the commentators were Adi Shankara, Ramanuja and Madhva (each of whom eventually set up sub-schools within Vedanta). Also, in the tradition of Purva Mimamsa, the author of the Sutra was Jaimini and the commentator was Shabaraswami.
Tibetan Buddhist scholars translate the term accurately as 'tenet'. In Tibetan Scholar Konchog Jigmed Wangpo's famous text on philosophical tenets, he writes:
The etymology for 'tenet' (siddhanta) is: a tenet or a meaning which was made firm, decided upon, or established in reliance on the texts and reasoning and which will not be forsaken for something else. Dharmamitra's Clear Words, A Commentary on Maitreya's Ornament for Realisations (abhisamayalamkara karika prajnaparamita mitopadesha shastratike) says: '"Established conclusion [siddhanta] signifies one's own established assertion which is thoroughly borne out by the texts and reasoning. Because one will not pass beyond this assertion, it is a conclusion."
For Jainism, the texts vary between the three primary sects, with Sthanakavasis believing in no textual authority. Both the Digambara and Shvetambara believe that the "purest" Jain teachings were contained within the Purvas, which have been mostly lost to antiquity. Of the surviving Jain scriptures, the Digambara tend to focus upon the Prakaranas; while the Shvetambara focus upon the Angas.
Early Indian astronomy is transmitted in Siddhantas: Varahamihira (6th century) in his Pancha-Siddhantika contrasts five of these: The Surya Siddhanta besides the Paitamaha Siddhantas (which is more similar to the "classical" Vedanga Jyotisha), the Paulisha and Romaka Siddhantas (directly based on Hellenistic astronomy) and the Vasishta Siddhanta.