In the Chandogya Upanishad, we see Svetaketu receiving spiritual instructions from his father. The father asked him to bring a fruit of the banyan tree. Then he said, ‘Split it into two.’ The boy did that. ‘What do you see?’ asked the father. ‘Seeds, small like particles,’ said the boy. ‘Break one of the seeds,’ said the father. ‘What do you see inside?’ ‘Nothing,’ said Svetaketu. ‘I tell you, a banyan tree is there,’ said the father. ‘The tree is there, hidden from your sight. But it is there. Even so, my dear boy, the Atman is invisible, but It exists in every being.’
The boy understood, but was not quite convinced. ‘Bring a glass of water,’ said the father. The son did that. ‘Now put a lump of salt in the water, put the glass aside and come tomorrow.’ The boy obeyed his father. The next morning the father said, ‘Bring now that glass of water. Drink a little water from the top. What do you find?’ ‘It tastes salty.’ ‘Now drink a little from the middle. What do you find?’ ‘It tastes salty, sir.’ ‘Now drink from the bottom.’ ‘It is also salty, sir.’ ‘Did you see the salt?’ ‘No, sir.’ ‘So is the Atman hidden in the body.’ (Chandogya Upanishad, 6.12.1-3.)
‘Quite concealed in all beings dwells the Atman even as butter dwells in milk. Ever churn, O aspirant, with mind as the churning stick.’ The bhakta always churns. He performs acts of devotion. And he obtains the butter and he is happy and contented. He is satisfied.