Two birds of beautiful plumage, inseparable companions, sat upon the same tree, one on the top and one below. The bird below ate the fruits of the tree, sweet and bitter. The moment it ate a bitter fruit, it was sorry, and after a while it ate another and when it was bitter too, it looked up and saw the bird above who ate neither the sweet nor the bitter, but was calm and majestic, immersed in its own glory. The lower bird went on eating the sweet and bitter fruits, until at last it ate one that was extremely bitter, and then it stopped again and once more looked at up at the glorious bird above. Then it moved closer to the glorious bird above; and when it was near enough, rays of light shone upon it and enveloped it. The bird saw that it was transformed into the higher bird. It became calm and majestic. In reality there had been but one bird all the time on the tree. The lower bird was simply a reflection of the one above.
Mundaka Upanishad