If there are no definable objects in the experiential world, and what we see around are the different stages of unending changes (Many), then what is that which undergo change? This is the cardinal question that perplexed Monists, because unending changes cannot happen in vacuum. A base platform is needed, upon which only changes can happen. What is this platform? From here on Metaphysics plays it role. Because, if everything in the experiential world is changing and thus undefinable, then any further inquiry on the subject (what is changing) cannot be conducted in the experiential world level. We must surpass it and think about trans-experiential level.
However I shall try to convince everyone the approach of the Upanishads towards the ‘Problem of One & Many’, by avoiding metaphysical speculations to a certain extend. The following conclusion is based on the Chandogya Upanishad where conversation between Uddalaka Aruni and his son Svetaketu takes place . Uddalaka instructs his son that, ‘by knowing (the characteristics) of a lump of earth, we can know the characteristics of everything made up of earth; by knowing the characteristics) of a lump of gold, we can know (the characteristics of) everything made up of gold’. This is a very important portion from Chandogya Upanishad because this portion touches and solves the ‘Problem of One & Many’.
We can summarize Uddalaka’s teachings as follows: –
— Names of all objects in the experiential world are merely conventional.
— Everything made up of gold, such as ring, necklace, etc., can be known, and therefore, reduce to the base substance ‘Gold’.
— In the same way, all other things, like Clay, Iron, etc., can be known and reduce to their respective base substances.
— At the very end, we will get 93 elements of physics as base elements.
(Science stop here. Everything is in physical layer up to this point. But Vedanta / Upanishads advances further to metaphysical layer and reduces these all 93 elements to a single Ultimate Reality.)
— The Upanishads reduces all 93 elements to a non-material and single substratum. The Upanishads call it as Prajna or Brahman. In English this can be translated roughly as Consciousness!
That is, material things of experiential world are reducible to a subjective, non-material Reality, which alone have Ultimate existence . In this way, ‘Many’ is included in ‘One’.
Upcoming article -> How the ‘Many’ arise from the ‘One’?
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 Chandogya Upanishad, Sankara Bhashyam. Page 409 onward. (Tran: Swami Gambhirananda, Advaita Ashrama. 2009).
 Ultimate existence – Existence without depending on anything other. Ultimate Reality can Exist by Itself without depending on any other things.