The authors of the Upanishads did not all think alike; but, taking their meditations as a whole, we may say that they are dominated by one paramount conception’ that of the ideal of oneness of the soul of man with the soul of the universe.
The Sanskrit word for the soul of man is Atman and the word for the soul of the universe is Brahman.
‘God’s dwelling place’, says Professor Radhakrishnan in this exposition of the philosophy of the Upanishads, ‘is the heart of man. The inner immortal self and the great cosmic power are one and the same.’
Brahman is the Atman, and the Atman is the Brahman. The one supreme power through which all things have been brought into being is one with the inmost self in each man’s heart.’
What is real in each of us is his self or soul. What is real in universe is its self or soul, in virtue of which its All is One, and the name for which in our language is God.
In other words of one of the Upanishads: ‘He who is the Brahman in man and who is that in the sun, these are one.’
Source: An Introduction by Edmond