कठोपनिषत् (1.20)

येयं प्रेते विचिकित्सा मनुष्ये- ऽस्तीत्येके नायमस्तीति चैके । एतद्विद्यामनुशिष्टस्त्वयाऽहं … ॥ २०॥

trans. (mine)

(Nachiketa to Yama)

"About the dead there is this doubt among men: “he is” says one; “he is not”, says another. I would know the truth of this, taught by you…"