Atman

However, we have been led beyond this to a threefold analysis which in its western version is body-soul-spirit and in its eastern version body-mind-atman.

Atman is the manifestation of brahman in the human body. The central theme of the Upanishads is that in the liberated state the atman is identical with brahman.[…]The word atman is generally translated as soul, self or spirit. However, in view of the Western definition of the soul, atman and soul are not the same.[…]In the Western view, the soul is created by God. In the Hindu view, the atman, being eternal, is not created by God. It is a part of God.

The human being in Hindu thought comprises Atman (or Punisha) and Maya (or Prakriti). The Hindu doctrine of Atman concerns one's deepest identity.

The priests who were immersed in the ritual science of the Brahmanas began to speculate on the nature of the self, and gradually the word "atman" came to refer to the essential and eternal core of the human person, which made him or her unique.

The Brahmanic tradition that Buddhism is both connected to and a reaction against was, according to almost every scholar, over the top as regards atman. So, not only were individuals possessed of an immutable, indestructible atman. Some, perhaps many Brahmins were asserting that they were ATMAN.