Chandogya Upanishad, 8:1.1 2 4.3. Translated by Eknath Easwaran

 The Self is hidden in the lotus of the heart. Those who see themselves in all creatures go day by day into the world of Brahman hidden in the heart. Established in peace, they rise above body-consciousness to the supreme light of the Self. Immortal, free from fear, this Self is Brahman, called the True. Beyond the mortal and the immortal, he binds both worlds together. Those who know this live day after day in heaven in this very life.

The Self is a bulwark against the confounding of these worlds and a bridge between them. Day and night cannot cross that bridge, nor old age, nor death, nor grief, nor evil or good deeds. All evils turn back there, unable to cross; evil comes not into this world of Brahman. One who crosses by this bridge, therefore, if blind, is blind no more; if hurt, ceases to be hurt; if in sorrow, ceases sorrowing. At this boundary night itself becomes day: night comes not into this world of Brahman. Only those who are pure and self-controlled can find this world of Brahman. That world is theirs alone. In that world, in all the worlds, they live in perfect freedom.