If all these “upanishads” have different authors, what is the underlying reason that they’re all classified/grouped together as “upanishads”?
It was a golden age where the rate of enlightenment was very high 🙂 After the study of the Vedas, many Rishi’s and Muni’s got enlightened, some by way of just listening to knowledge and some by deep sadhana. Upanishads were an enumeration of sanskrit shlokas by these enlightened Rishi’s and Muni’s, who experienced this knowledge in deep meditation. They taught these to earnest disciples. As this knowledge was the highest knowledge, about the Consciousness/Brahman/God, it was preserved together as Upanishads, also called Vedanta (the end of the Vedas). It is called the end because, one finds God in the last place, and the Upanishads search ends there!
The “Om Sahana Vavatu” prayer – did this originate from Katopanishad? Or did we recite it every time because of its “together” theme, to emphasize our shared pool of consciousness?
It is the Shanti mantra that is a part of all the Upanishads. All Upanishads begin with this prayer.
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