Forest in the highlands of Kerala, South India
Maitreyi was confused about the idea of a Self in which individuality is dissolved. She asked her husband, the sage Yagnavalkya, to clarify. He replied: "As long as the dualism of the mind is operating, one senses and knows things and people as others, but for those who have realised the Self, in which all things are dissolved, who is there to be sensed by whom, who is there to be known by whom? By whom shall the Senser be sensed? By whom shall the Knower be known? The Self is described as not this, not that. It is incomprehensible, for it cannot be grasped by the tongs of dualism."
(Adapted from the Brhadaranyaka Upanishad by Jeronimus )
The Bṛhadāraṇyaka Upanishad (which translates as the Great Forest Teaching) is one of the oldest of the Upanishads (possibly the oldest), dating to approximately the 8th to 7th centuries BCE. It figures as number 10 in the Muktika canon of 108 Upanishads and was notably commented upon by Adi Shankaracharya. T S Eliot based one of his poems on it.