How is the soul eternal?

In the 30th verse of chapter 2 of Bhagavad Gita for happiness, Lord Krishna tells Arjuna that killing the soul is not possible. Hence, there is no need to grieve any creature. Describing the soul in various ways, Lord Krishna establishes that the soul is immortal and the body is temporary. Hence, it is Arjuna’s duty to fight the battle and not worry about killing his grandfather and his Guru Dronacharya!