Chandogya Upanishad
Harih, Om. There lived once Svetaketu Âruneya (the grandson of Aruna). To him his father (Uddâlaka, the son of Aruna) said: Svetaketu, go to school; for there is none belonging to our race, darling, who, not having studied (the Veda), is, as it were, a Brâhmana by birth only.'
Having begun his apprenticeship (with a teacher) when he was twelve years of age, Svetaketu returned to his father, when he was twenty-four, having then studied all the Vedas,—conceited, considering himself well-read, and stern.
His father said to him: Svetaketu, as you are so conceited, considering yourself so well-read, and so stern, my dear, have you ever asked for that instruction by which we hear what cannot be heard, by which we perceive what cannot be perceived, by which we know what cannot be known?'
'What is that instruction, Sir?' he asked. The father replied: 'My dear, as by one clod of clay all that is made of clay is known, the difference being only a name, arising from speech, but the truth being that all is clay;
'And as, my dear, by one nugget of gold all that is made of gold is known, the difference being only a name, arising from speech, but the truth being that all is gold?
'And as, my dear, by one pair of nail-scissors all that is made of iron (kârshnâyasam) is known, the difference being only a name, arising from speech, but the truth being that all is iron,—thus, my dear, is that instruction.'
The son said: 'Surely those venerable men (my teachers) did not know that. For if they had known it, why should they not have told it me? Do you, Sir, therefore tell me that.' 'Be it so,' said the father.