Āruṇi, son of Aruṇa Aupaveśi, is one of the more important teachers mentioned in the Upaniṣads. When he was under-going training in Vedic studies, his guru Dhaumya once commanded him to stop the water that was leaking out of his fields. Āruṇi lay across the breach and stopped it. Hence, he came to be known as Uddālaka (‘One who bound or fastened the bund’). Gautama is another appellation for him. Śvetaketu was his son.
He once went to the king Pravāhaṇa Jaivali and was humble enough to learn about the Pañcāgnividyā or the ‘doctrine of five fires’ from him.
When he discovered that his son Śvetaketu was puffed up with pride after returning from his teacher’s house, due to Vedic learning, he put him a fundamental question: ‘Have you known that by knowing which, all the unknown becomes known?’ Since Śvetaketu did not know and requested him to teach it, Āruṇi taught him this esoteric wisdom. (vide Chāndogya Upaniṣad, chapter 6.)