'But when his speech is merged in his mind, his mind in breath, breath in heat (fire), heat in the Highest Being, then he knows them not. 'That which is the subtile essence, in it all that exists has its self. It is the True. It is the Self, and thou, O Svetaketu, art it.' 'Please, Sir, inform me still more,' said the son. 'Be it so, my child,' the father replied.

In plain words: Once his speech, mind, breath and heat merge back into the Highest Being, he no longer knows them. That same hidden essence is the real self of all, and Svetaketu is that. The son asks to learn more, and the father agrees.