Sri Aurobindo translates Chapter 2, Section 2, Verses 5-7 of the Mundaka Upanishad: “He in whom are inwoven heaven and earth and the mid-region, and mind with all the life-currents, Him know to be the one Self; other words put away from you: this is the bridge to immortality. Where the nerves are brought close together like the spokes in the nave of a chariot-wheel, this is He that moves within,– there is He manifoldly born. Meditate on the Self as OM and happy be your passage to the other shore beyond the darkness. The Omniscient, the All-wise, whose is this might and majesty upon the earth, is this Self enthroned in the Divine city of the Brahman, in his ethereal heaven.”
While still couched in language that could conceivably be interpreted in some external sense, these verses clearly represent the inner psychological reality and the process of realisation of the unity of the Brahman as the universal creation with the internal experience of the individual seeker. Heaven, earth and the mid-region refer not just to some external world-reality, but to the mental, physical and vital planes within the human individual. Other Upanishads, such as the Taittiriya, provide a key to understanding such terms as psychological states, not just physical realities. The connection to the meditation on OM is further evidence that these verses are intended as an instruction to a disciple or student on how to attain the realisation of the unity of Brahman as the entire creation, and beyond, that was stated in verse 4.
The Taittiriya Upanishad summarizes the unity of the Self in the individual and the Self in the universe as follows: “The Spirit who is here in man and the Spirit who is there in the Sun, lo, it is One Spirt and there is no other.” (Sri Aurobindo, The Upanishads, Taittiriya Upanishad, Bhriguvalli, Ch. 10, pg. 281)
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210