Om Swami
How can I stay unshaken by ups and downs of happiness and sorrow?
The witness bird within
The Upanishad speaks of two birds in us: one eats the fruit and becomes happy or unhappy, the other only watches. The one living as the witness (sakshi bhav) has no agitation rising in its mind and is not sad at all.
Now the bird eating the fruit, and the other that has become a witness, in witness mode, merely watching, has no agitation of any kind arising in its mind.