The force that changes matter cannot reach an intangible soul.
It is said that weapons cannot cut the soul, fire cannot burn it, water cannot wet it, and wind cannot dry it. In annotations, these represent material forces corresponding to the four elements of earth, water, fire, and wind.
If it is the body, it can be cut, burned, wet, and dried. However, the intangible soul itself cannot be reached by forces that change matter. Arjuna was afraid that weapons could even destroy the very existence of his relatives. In contrast, specific examples demonstrate that no material force can destroy the soul.
Japanese Translation from the Geeta Press Edition - Chapter 2, Section 23: "The soul is not cut off, nor is it burned, nor is it moistened, nor is it dried."
After demonstrating that it is unreasonable to grieve the process of leaving one body and taking another, the master indirectly discusses again the eternal and immutable nature of the soul in the following three sections.
This is because the nature of the soul is difficult to understand, and proving that grieving the destruction of the soul is inappropriate and unsuitable.
Sanskrit original text
नैनं छिन्दन्ति शस्त्राणि नैनं दहति पावकः ।
न चैनं क्लेदयन्त्यापो न शोषयति मारुतः ॥ २३ ॥Romanization
nainaṃ chindanti śastrāṇi nainaṃ dahati pāvakaḥ |
na cainaṃ kledayanty āpo na śoṣayati mārutaḥ || 23 ||The weapon cannot cut it, and fire cannot burn it. Water cannot wet it, and wind cannot dry it. (23)
Arjuna's sorrow stemmed from the anxiety that he might have to kill elders and other relatives by using deadly weapons, or that he might have to throw destructive weapons at them.
Therefore, in order to remove that sorrow, the Lord shows that all four elements – earth, water, fire, and wind – cannot destroy the soul, thus establishing the immortality and immateriality of the soul.
The Lord is showing that even if the body is cut by weapons, the soul is not affected.
Destructive fire weapons may burn the body. However, the soul will not be burned. The water weapon called "Vāruṇāstra" might be used to melt the body. However, it will not melt the soul. The wind weapon called "Vāyavyāstra" might dry out the body. However, it will not dry out the soul.
The body is something that perishes and has a form. The soul is something that endures and does not have a form. Therefore, the soul will never be destroyed by earthly elements that take the form of weapons, nor by water, fire, or wind elements.