Upanishads
Ātman is a central idea in all the Upanishads, and “Know your Ātman” their thematic focus. These texts state that the core of every person’s self is not the body, nor the mind, nor the ego, but Ātman – “Soul” or “Self”. Atman is the spiritual essence in all creatures, their real innermost essential being. It is eternal, it is the essence, it is ageless.