Chapter 6 of the Srimad Bhagavad Gita is titled, “The True Yoga”. In this chapter, the teacher, Shri Krishna, teaches his pupil, Arjuna, the nature of True Yoga. It begins with the explanation of the true nature and spirit of renunciation, and the relationship of renunciation to desireless action.
The Atman cannot be cut, burnt, suffer any decay or be dried out. The five forces of nature which are capable of destroying physical elements by their combined actions have no influence on the Atman. That is why it is referred to as nitya or permanent or unchangeable.
The soul or Atman casts off its worn out physical body and takes residence in a newer physical body at regular intervals in time. Just like the clothes that we wear do not represent the real state of our physical body, the state of our physical body (which deteriorates over time and is hence impermanent) does not represent the truly unchangeable and permanent nature of the soul that resides within it.