Aruni (c. 8th century BCE), also referred to as Uddalaka or Uddalaka Aruni, is a revered Vedic sage of Hinduism. He is mentioned in many Vedic era Sanskrit texts, and his philosophical teachings are among the center piece in the Brihadaranyaka Upanishad and Chandogya Upanishad, two of the oldest Upanishadic scriptures. A famed Vedic teacher, Aruni lived a few centuries before the Buddha and attracted students from far regions of the Indian subcontinent; some of his students such as Yajnavalkya are also highly revered in the Hindu traditions. Both Aruni and Yajnavalkya are among the most frequently mentioned Upanishadic teachers in Hinduism.
According to Ben-Ami Scharfstein, a professor emeritus of Philosophy at Tel Aviv University, Uddalaka Aruni was one of the first philosophers in recorded history. In the Chandogya Upanishad Aruni asks metaphysical questions concerning the nature of reality and truth, observes constant change, and asks if there is something that is eternal and unchanging. From these questions, embedded in a dialogue with his son, he presents the concept of ?tman (soul, Self) and universal Self.
In sections 3.7 and 6.2 of the Brihadaranyaka Upanishad, in a dialogue where Aruni is relatively a minor participant.
In sections 6.1-16 and 5.3 of the Chandogya Upanishad, as a major dialogue between Aruni and his son Svetaketu, a dialogue about Atman and Brahman that contain ideas foundational to the Vedanta school of Hindu philosophy. The dialogue sets the context of the son, who goes to a Vedic school for twelve years of studies, is conceited that he has learnt the books (Vedas). Aruni, the father enquires and presses Svetaketu whether at school, he pondered and understood the nature of existence, what is truth, what is reality, the meaning of life and self-knowledge, and the relationship between oneself, other beings and the universal self.
In verse 1.1 of the Kaushitaki Upanishad, where the scion of Gangya invites Aruni, but he sends his son to the event. This verse is notable for the conversation therein that suggests the full name of Aruni to be Uddalaka Aruni Gautama, and the mention of him as one of the characters in a group event that hosted "Vedic studies in the hall of sacrifice" (yajna).
In the Katha Upanishad, which opens with the story of Vajasravasa, also called Aruni Auddalaki Gautama. According to Max Muller - an Indologist at Oxford University, assuming the manuscripts have been correctly reproduced over their history, there may be a difference between "Auddalaki" (grandson) and "Uddalaki" (son), but he adds Adi Shankara considered them to be same Aruni, in his commentaries on the Upanishads. The theme discussed in the dialogues of the Katha Upanishad is also Atman and Brahman. Paul Deussen, an Indologist at University of Kiel, states that there are inconsistencies about his full name in the Hindu traditions.
Sage Aruni is revered in the Hindu tradition, and like many of its revered ancient scholars, later era scholars from the earliest times attributed or named their texts after him. Some of these treatises include:
Arunisruti, also called Uddalaka Sruti, likely a medieval era theistic text that has been lost to history, and one cited by Madhvacharya.
Aruni Upanishad, also called Aruneya Upanishad, is one of the oldest renunciation and monk life-related Sannyasa Upanishads of Hinduism. The text was likely completed in or after 4th-century BCE but before the start of the common era, according to Joachim Sprockhoff, the German scholar of Upanishads and according to Patrick Olivelle. The Aruni Upanishad states that bookish and ritual knowledge is irrelevant, the true pursuit of knowledge is the meaning, the essence and the import of Vedic ideas, one has the right and duty to abandon the worldly life in the singular pursuit of spirituality.
Uddalaka Aruni is said to have systematised the Vedic and Upanishadic thoughts. Many Mahavakyas are ascribed to sage Uddalaka Aruni. Among those, "Tat Tvam Asi" (That thou art) of the Chandogya Upanishad is an oft quoted thought in Hinduism. Its teacher is Uddalaka Aruni and the student his son Svetaketu.
His teachings extend beyond metaphysical speculations and philosophy. Parts of his works contain the seeds of Indian atomism, because of his belief that "particles too small to be seen mass together into the substances and objects of experience". Some scholars such as Hermann Jacobi and Randall Collins have compared Aruni to Thales of Miletus in their scientific methodology, calling them both as "primitive physicists" or "proto-materialist thinkers".