The hint is given: the something which is spread all over the body, that is eternal. And what is that something? That something is our consciousness. Here it is stated, avināśi tu tad viddhi yena sarvam idaṁ tatam. In this body there is something. That is consciousness. That is eternal. Just like if you or I pinch my body, I feel pain because the consciousness is there. But when the consciousness will not be there, if I cut my hand or cut your hand, you'll not protest. Even scientists have proved this consciousness is there in the tree also. If you cut the tree, there is sensation, feeling some pain, and this is recorded in the machine. So here it is hinted that this consciousness is spread all over the body, that is eternal. The body is not eternal. As soon as the consciousness is gone, the body is dead. Therefore we should take care of the thing which is cons... That is the soul. On account of presence of the soul, there is consciousness.
So Kṛṣṇa further says in this connection, antavanta ime dehā nityasyoktāḥ śarīriṇaḥ (BG 2.18). This body... Deha means body. Antavat, it is by this material body, that is eternal. So that consciousness, or the rays of the soul, is described here. Na jāyate na mriyate vā kadācit. "This consciousness of the soul is never born, neither it is ever dead." Nāyaṁ bhūtvā bhavitā vā na bhūyaḥ. The soul and the consciousness has no past, present, or future. It is eternal. Aja. Aja means who does not take birth. Ajo nitya, eternal; śāśvata, ever-existing; ayaṁ purāṇa, the oldest. Na hanyate hanyamāne śarīre (BG 2.20). When the body is annihilated, the soul and consciousness is not annihilated. Just like when we sleep, our consciousness works in a different body, subtle body: mind, intelligence and ego. That we have got experience every night. We sleep on our bed, but my consciousness goes to other country or other place, and work in a different way.