18. These bodies have end, even though the Indweller is said to be Eternal, Indestructible, and Incomprehensible. Therefore, fight (as the form of worship) O Arjuna.
Bhashya :- This verse speaks that there is nothing like eternal existence for the body. Even as with the destruction of the mirror the reflection is also destroyed, but not so with the destruction of the Self. Iit is not so, because Self is called as Eternal. शरीरिणा: means the Lord, the Indweller. There is no destruction for him in the ordinary sense of meaning. Because he is referred as Indestructible. Why? Because of the incomprehensible nature of the Supreme Lord. But (in his case) there is no उपाधि limitation, as in the case of the destruction of the mirror, where the reflection is destroyed. Here the Self is the Reflector through the medium of Mind. There exists no उपाधि limitation for the Eternal One. Thus says the lord. For the Realized One after deliverance, Eternal is the form of his Self. Thereafter, form of consciousness of the Jiva becomes the reflection of Keshava.
Tatparya Nirnaya :- Since without choice, the body has other attributes; it is covered in pain, thus in Parama Shruti. Its (body's) destruction in war is also a form of worship of the Incomprehensible Lord. It (destruction of the body being under His control, (it leads to the) relief from both pain and pleasure. “By surrendering all actions to Me and devoted to Me, you shall be protected by Me”. By such words (of the Lord).Spoken as eternal referring the Jiva, and the word अविनाशी is repeated, then, the statement "Know that by Indestructible all this is enveloped" and the words अविनाशीनो`maoyasya – indestructible, incomprehensible being in reference to the Lord. By specifying इमे देहा, the eternal transcendental form of Consciousness and Blissful state of the realized souls, is made known.