Sri Aurobindo translates Katha Upanishad, Second Cycle: Second Chapter, Verses 9-12: “Even as one Fire has entered into the world, but it shapes itself to the forms it meets, so there is one Spirit within all creatures, but it shapes itself to form and form: it is likewise outside these. Even as one Air has entered into the world, but it shapes itself to the forms it meets, so there is one Spirit within all creatures, but it shapes itself to form and form: it is likewise outside these. Even as the Sun is the eye of all this world, yet is not soiled by the outward blemishes of the visual, so there is one Spirit within all creatures, but the sorrow of this world soils it not: for it is beyond grief and his danger. One calm and controlling Spirit within all creatures that makes one form into many fashions: the calm and strong who see Him in their self as in a mirror, theirs is eternal felicity and ’tis not for others.”
All of the forms we observe in the world are of the substance of one Spirit; there is no other existence than this One. Western science recognises that all material forms consist of atoms, which differentiate themselves based on the internal configuration of numbers of protons, electrons, neutrons, etc. They already recognise that matter is convertible to energy. They also recognise, through the processes of nuclear fusion and nuclear fission, that one element can be transformed to other elements. This provides a basis for understanding the Oneness spoken of in the Upanishad.
At a more outward level, we see that air is the same for all creatures. Each creature takes from it the elements it can use, and returns to the air the elements it cannot use. In the natural process of Nature, we then see that all living things on the planet are symbiotic. Plants take up the carbon dioxide and excrete oxygen. Animals take up the oxygen and excrete carbon dioxide. There is a balance, a homeostasis that pervades all Nature, and if the balance between plants and animals is upset, or other elements, created artificially, change the balance of the components of air, there can be enormous consequences.
The question arises, if there is one Spirit that embodies itself in all these forms, does it take on the burden of deficiencies, failures, sins, if you will, of specific forms or beings. The example of the Sun is given that, while it is also one with all of creation, it carries out its function without becoming itself negatively impacted by the successes or failures of those upon which it shines.
The universal Observer, the universal Existent, takes on all the various forms, constitutes them, and is yet transcendent to them, and does not suffer under the weaknesses of any individual embodiment. The ego-personality attached to a specific being experiences joy and sorrow, pain and pleasure. The divine Being is free of the attachment that brings about the suffering of the dualities. The individual who is able to shift his awareness to the divine standpoint similarly escapes the bonds of the ego and the consequent suffering.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129