Sri Aurobindo translates Taittiriya Upanishad, Shikshavalli, Chapter 7: “Earth, sky, heaven, the quarters and the lesser quarters; Fire, Air, Sun, Moon and the Constellations; Waters, herbs of healing, trees of the forest, ether and the Self in all; these three concerning this outer creation. Then concerning the Self. The main breath, the middle breath, the nether breath, the upper breath and the breath pervasor; eye, ear, mind, speech and the skin; hide, flesh, muscle, bone and marrow. Thus the Rishi divided them and said, ‘In sets of five is this universe; five and five with five and five He relateth.”
The Rishi of the Upanishad does not abandon the outer world, but looks upon it in a spirit of examination and discovery. The powers of the human mind are to be applied, not denied. These powers include observation, categorisation and analysis. The students are being asked to use these powers to understand the nature of the world around them. This helps them train the mind and prepares them for the deeper examinations that must follow to discover the spiritual truths of our existence. The Self referred to here is the outer being that moves in and relates to the outer world.
The classifications that assemble various items together also helps the seeker to understand the inter-connectedness of the forms and forces in the world. For example, the 5 breaths, or Pranas, mentioned do not exist independently of one another but are clearly related and dependent of the action of one upon another. We breathe in, and out. The energy of the breathing permeates and pervades the entire body. Specific motions of the life energy carry out various bodily functions. These are forms of Prana and all are needed to form a functioning organism.
The method of categorisation also helps the seeker to appreciate the layers that work together, as the example of the skin is used to take the seeker successively inwards from hide, to flesh, to muscle, to bone to marrow. Not only does this show an understanding of anatomy, but trains the seeker to find the deeper levels that are not apparent on the surface of things. Exercising this capacity becomes important as the Upanishad moves to the more direct teachings in the Brahmanandavalli and Bhriguvalli which follow.