Chandogya Upanishad 6.42

अथ यत्रैतत्पुरुषः पिपासति नाम तेज एव तत्पीतं नयते तद्यथा गोनायोऽश्वनायः पुरुषनाय इत्येवं तत्तेज आचष्ट उदन्येति तत्रैतदेव शुङ्गमुत्पतितँ सोम्य विजानीहि नेदममूलं भविष्यतीति ॥ ५ ॥

6.8.5 — atha yatraitatpurushah pipasati nama teja eva tatpitam nayate tadyatha gonayo'shvanayah purushanaya ityevam tatteja aachashta udanyeti tatraitadeva shungamutpatitan somya vijanihi nedamamulam bhavishyatiti || 5 ||

'When a man is thus said to be thirsty, fire carries away what has been drunk by him. Therefore as they speak of a cow-leader (go-nâya), of a horse-leader (asva-nâya), of a man-leader (purusha-nâya), so they call fire udanyâ, thirst, i.e. water-leader. Thus (by water digested &c), my son, know this offshoot (the body) to be brought forth: this (body) could not be without a root (cause).

'जब पुरुष को "पिपासा" (पीने की इच्छा) होती है, तब पिया हुआ (जल) तेज ले जाता है। जैसे गोनाय, अश्वनाय, पुरुषनाय — वैसे ही तेज को "उदन्या" (जल-नायक, तृषा) कहते हैं। हे सौम्य, जान कि यह शरीर-रूपी शाखा यों उत्पन्न हुई है — यह बिना मूल के नहीं हो सकती।'