I am eternal, the body is not eternal. This is the fact. Antavanta ime dehā nityasyoktāḥ śarīriṇaḥ (BG 2.18). Śarīriṇaḥ, the owner of the body, is eternal, but not the body. In so many ways Kṛṣṇa is explaining about the material condition of this body. But those who are not very intelligent, with poor fund of knowledge, it is very difficult for them to understand. Otherwise, things are very clear. This point is very clear. That at night I forget this body, and in daytime I forget the body at night. This is a fact. Similarly, I may forget the body of my last appearance, last duration of life, or I may not know the future body. But I will exist, and the body may change, but I'll have to accept another body, which is temporary. But I, as I exist, it means I have got a body. That is spiritual body.
So spiritual body is existing, and spiritual advancement means first of all to know spiritual identification of myself. Just like Sanātana Gosvāmī went to Śrī Caitanya Mahāprabhu after retiring from his ministership. So he first of all said that ke āmi, kene āmāya jāre tāpa-traya (CC Madhya 20.102): "Actually, I do not know what I am, and why I am subjected to the miserable condition of life." Therefore the miserable condition of life is this body. Because I get . . . in dream also. When I get another body, sometimes we find that on top of the very tall bamboo or tall mountain, I am just now, I'm falling down. And I'm afraid, I sometimes cry, "Now I am now falling down." So this body, this material body, which body I belong to, which I am . . . actually, I do not belong to any of these bodies. I have got a separate spiritual body.
So this human life is meant for that realization that, "I am not this material body. I am spiritual body." Then next question will be, "Then what is my function?" In the present body, under some material condition I am thinking, "This is my body," and the body is produced under certain condition of this country or this family; therefore, "This is my family, this is my country, this is my nation." Everything in bodily concept of life. And if I am not this body, then in relationship with this body, either my family or my country or my society or my other relationship, they are also false, because the body is false.
Therefore Śaṅkarācārya theorized this, brahma satyaṁ jagan mithyā. Brahman means the soul is actually the fact, not the material manifestation. Material manifestation, of course, he says false. We don't say false. We say temporary. So our main concern is that I am not temporary; my body is temporary. Now I am working for the body. That is illusion. Ahaṁ mameti (SB 5.5.8). Then what is real fact? Real fact is that I am spiritual particle, and the whole spirit is Kṛṣṇa, or God. Therefore, as part and parcel of God, it is my duty to serve God. That is spiritual life, bhakti-yoga. That is called svarūpa. And in other place, the Bhagavad-gītā confirms it that sa guṇān samatītyaitān brahma-bhūyāya kalpate (BG 14.26). When I realize that I am not this body, then immediately I transcend the three modes of material nature: sattva-guṇa, rajo-guṇa, tamo-guṇa. Under the bodily concept of life I am influenced by one of the modes of material nature and acting.