Chandogya Upanishad 8.4.1

In this world there are so many varieties of things, and each has its role to play. Human beings have their role to play; animals have theirs, and plants have theirs. The sun rises at a certain time. It never fails. Each thing is in its place, doing what it’s supposed to be doing. There should be no mix-up. If there were a mix-up, there would be chaos. Who supervises all this so that everything is in its proper place? It is the Self.

The word setu usually means ‘bridge,’ but here it means a dam. Suppose you have a large river and you want to separate the water for some reason or other. You then erect a dam. This keeps the two sides apart and prevents them from mixing. Similarly, the Upaniṣad says, in this phenomenal world the Self acts as a dam so that everything functions as it should, without getting mixed up. The Self stands as a barrier, keeping each thing in its place. It is never failing. Day and night, sorrow, sin, the castes and stages of life, material things—animate or inanimate—whatever there is in the world of diversity,

It is the Self that creates all the diversity—good, bad, rich, poor, educated, ignorant. There are so many kinds of people and so many kinds of plants, animals, and objects. The Self not only manifests all this diversity—it also maintains it. It does not want the diversity to disappear, because this diversity is necessary for the phenomenal world to go on.

But these divisions do not touch the Self. Nothing can affect it. It is the Master, controlling everything, but it is not controlled by anything. Mṛtyu (death), for instance, is so powerful. Everyone is subject to death, but it cannot overcome this dam, the Self. No blemish or impurity can even approach the Self. It is never affected by the good or bad that exists in the world. It is always the same—constant and pure.

The Self is also called here brahma-loka. The Self, the ātman, is Brahman. When you look at the world, you say, ‘Sarvam khalu idam brahma’—all this is Brahman. When you look inside, within you, you say, ‘Aham brahmāsmi’—I am Brahman, or ‘Ayam ātmā brahma’—this self is Brahman. It is the same Self, inside and outside.

Śaṅkara says that if you know you are the Self, you are not affected by anything. But how do you realize the Self? Śaṅkara says you realize it by brahmacarya, by continence and self-control.