Then Uddalaka Aruni asked. 'Yagnavalkya,' he said, 'we dwelt among the Madras in the houses of Patankala Kapya, studying the sacrifice. His wife was possessed of a Gandharva, and we asked him: "Who art thou?" He answered: "I am Kabandha Atharvana." And he said to Patankala Kapya and to (us) students: "Dost thou know, Kapya, that thread by which this world and the other world, and all beings are strung together?" And Patankala Kapya replied: "I do not know it, Sir." He said again to Patankala Kapya and to (us) students: "Dost thou know, Kapya, that puller (ruler) within (antaryamin), who within pulls (rules) this world and the other world and all beings?" And Patankala Kapya replied: "I do not know it, Sir." He said again to Patankala Kapya and to (us) students: "He, O Kapya, who knows that thread and him who pulls (it) within, he knows Brahman, he knows the worlds, he knows the Devas, he knows the Vedas, he knows the Bhutas (creatures), he knows the Self, he knows everything." Thus did he (the Gandharva) say to them, and I know it. If thou, O Yagnavalkya, without knowing that string and the puller within, drivest away those Brahma-cows (the cows offered as a prize to him who best knows Brahman), thy head will fall off.' Yagnavalkya said: 'O Gautama, I believe I know that thread and the puller within.' The other said: 'Anybody may say, I know, I know. Tell what thou knowest.'

In plain words: Uddalaka tells how a spirit once asked his teacher about the hidden thread that ties this world, the next world and all beings together, and about the inner ruler who guides them from inside. He challenges Yagnavalkya to explain these, or else lose his head for taking the prize cows. Yagnavalkya says he knows, and Uddalaka says anyone can claim that, so prove it.