Chandogya Upanishad 6.2.3

The scriptures say that this Existence, or Brahman, is one without a second, always the same, unchanging, and unchangeable. So how can it at the same time be many? Vedānta says that it is like a lump of earth sometimes taking different forms of pots, cups, etc. Or it is like a rope which sometimes looks like a snake. The change is only in names and forms. It is not a real change.

Vedānta says, this applies to all that we see in this world. Underlying the seeming diversity there is unity. This unity—this One—supports the diversity. It is this One that has become many, but only in appearance. To go back to the example of the earth, the earth remains earth, though in appearance it may take the form of a lump or of some pots or of some other things.

An apt description of this is ‘unity in diversity.’ It is Brahman that has become everything—space, air, fire, water, earth.

Then the question arises, how can fire, which is inanimate, think? The answer is that here fire stands for Brahman. Brahman as fire is making things happen. Brahman is the source of everything. It is the base, the reality. Just as earth can be in the form of a lump or a pot, similarly, Brahman can be in the form of fire, water, or something else.

It is not that Brahman undergoes any change. Brahman remains Brahman. It is Sat, eternal Existence. It is the Absolute. It is never conditioned by anything. When it says it will be ‘many,’ it means it will appear in many forms, just as earth or gold can be in many forms. But it must be borne in mind that this Existence is also pure consciousness.