Directly we cannot appreciate what is that spiritual fragment, particle, which is within this body. Because the length and breadth of that spirit soul is impossible to be measured by our material instruments, although the scientists say that we can measure it. Anyway, even it is possible, first of all, you have to see where the soul is situated. Then you can attempt to measure it.
So first of all, you cannot see even, because it is very, very small, one ten-thousandth part of the tip of the hair. Now, because we cannot see, by our experimental knowledge we cannot appreciate. Therefore Kṛṣṇa is describing the existence of the self, soul, in a negative way: "It is not this." Sometimes when we cannot understand, the explanation is given, "It is not this." If I cannot express what it is, then we can express in a negative way that, "It is not this."
So what is that "not this"? The "not this" is that, "It is not material." The spirit soul is not material. But we have got experience of material things. Then how to understand that it is the negative? That is explained in the next verse that, nainaṁ chindanti śastrāṇi: you cannot cut the spirit soul by any weapon, knife, sword or pistol. It is not possible. Nainaṁ chindanti śastrāṇi. The Māyāvāda philosophy says that, "I am Brahman. Due to my illusion, I feel I am separated. Otherwise, I am one." But Kṛṣṇa says that mamaivāṁśo jīva-bhūtaḥ (BG 15.7). So does it mean that the, from the whole spirit, this fragment has been separated by cutting into piece? No. Nainaṁ chindanti śastrāṇi. It cannot be cut into pieces.