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Summary - Read the conclusion at the bottom for short answer, where various categories of Shastras - Upanishads, Grihya/ Dharma Sutras, Puranas, Itihasa; unanimously prescribe the wearing of the Yajnopavita and Shikha while following Sannyasa. When so many categories of shastras are saying the same thing, there can be no bigger Shastra pramana.
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Indologist Nrisinghaprasad Bhaduri opined that "Vanaras" mentioned in the Valmiki Ramayana were not monkeys. They were non-Aryan races. In support of his view, he mentioned the following points.
In Sanskrit, "Vanara" means "human-like".
A few members of Vanaras knew Vedas, Arthashastras, Ayurvedas and even a bit of politics. ...
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