अन्तवन्त इमे देहा नित्यस्योक्ताः शरीरिणः ।
अनाशिनोऽप्रमेयस्य तस्माद्युध्यस्व भारत ।।
antavanta ime dehā nityasyoktāḥ śarīriṇaḥ ।
anāśino'prameyasya tasmādyudhyasva bhārata ।।
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1. Our body is called 'deha'. The very meaning of 'deha' means to grow. That which grows (upacaya) will shrink (apacaya). Growing/Shrinking happens to those things that have parts (avayava). Any thing that has avayava will surely (niyamena) be destroyed
3. Once punya/papa karmas are experienced by our deha, it completes it's purpose & gets destroyed.
Svami Desikan in tAtparya-candrika has given 6 (primary) reasons for this
2. By nature, Atma & Deha have so much of differences. Deha is not Atma
3. There's nothing that stops deha from perishing
4. Even lightning looks stable when compared to deha
6. We do not know when deha will perish, hence all of us must strive for moksha-prapti sadhana called prapatti (Surrendering to Him) immediately
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By this shloka, 'asat' padArtha is described. Deha of nityAtma gets destroyed as seen in dreams or magic..
We see water in dreams/magic. But when we wake up, we know for sure there's nothing like that. (pratyakshAdi pramana).
The same way sharira also appears/disappears
Both these changes are not observed in nitya-shuddha-Atma. The deha is just a mirage that envelops Atma & will quickly dissipate when it is discerned
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Just as Limitless, All Pervading paramAtma is Eternal, so are jIvAtmas. It is just that these jIvAtmas are enveloped in a destructible sharira that is causing false perceptions for us. Hence, don't worry Arjuna. We all are nityas. Be fearless & go for the battle!!