Mandukya Karika, verse 3.21-22
न भवत्यमृतं मर्त्यं न मर्त्यममृतं तथा ।
प्रकृतेरन्यथाभावो न कथंचिद्भविष्यति ॥ २१ ॥
na bhavatyamṛtaṃ martyaṃ na martyamamṛtaṃ tathā |
prakṛteranyathābhāvo na kathaṃcidbhaviṣyati || 21 ||
21. The immortal cannot become mortal, nor can the mortal ever become immortal. For, it is never possible for a thing to change its nature.
स्वभावेनामृतो यस्य भावो गच्छति मर्त्यताम् ।
कृतकेनामृतस्तस्य कथं स्थास्यति निश्चलः ॥ २२ ॥
svabhāvenāmṛto yasya bhāvo gacchati martyatām |
kṛtakenāmṛtastasya kathaṃ sthāsyati niścalaḥ || 22 ||
22. How can he, who believes that the naturally immortal entity becomes mortal, maintain that the Immortal, after passing through change, retains its changeless nature?
Shankara Bhashya (commentary)
As in common experience the immortal never becomes mortal, nor the mortal ever becomes immortal; therefore it is, in no way, possible for a thing to reverse its nature, i.e., to become otherwise than what it is. Fire can never change its character of being hot. The disputant who maintains that the naturally immortal entity becomes mortal, i.e., really passes into birth, makes1 the futile proposition that that entity before creation is by its very nature, immortal. How can he assert that the entity is of immortal nature if it be admitted that it passes2 into birth? That is to say, how can the immortal retain its immortal nature of changelessness if it should undergo a change? It cannot, by any means, be so. Those3 who hold that the Ātman passes into birth (i.e., undergoes a change), cannot speak of the Ātman as ever birthless. Everything, according to them, must be mortal. Hence4 there cannot be a state called liberation.