To effectively convey the meaning of the supra-mundane state in ordinary language, the Upanisadic thinkers projected the state of liberation – the real happiness – as great Self (Ātman) as well as the eternal Brahma. When one renounces or contains all desires, one becomes great Self (Ātman)/Brahma.
The Upanisadic thinkers also taught that one’s ‘inner soul’ (antarātman) is in fact the great Self/Brahma, and that one can discover it. The poet of Kaṭha Up 5.12 supported the theory of an original pure soul and asserted that the ‘Inner Soul (antarātman)’ is in all things and that those wise people who can see it have eternal happiness.
This poet added that this inner soul is ‘the Constant among the inconstant, the Intelligent among the intelligences’ (KaṭhaUp 5.13).27 That is, there is a permanent self in impermanent beings. This idea – the inner self is pure and it must be realised or recovered – was also shared by the Buddha. He told his monks that the original mind (citta) is luminous but is defiled by taints and is to be cleansed (AN I 10)
However, the Upanisadic thinkers and the Buddha do not share the same terminologies. The Upanisadic thinkers named the original pure mind as ‘inner soul’. The Buddha felt that since the original mind is pure and does not have any mundane or worldly feeling, it cannot be termed as ‘self’