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.
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.