The following is taken from the Katha Upanishad, a teaching from Yama to his pupil Nachiketas:
Even by the mind this truth is to be learned; there are not many but only one. Who sees variety and not the unity wanders on from death to death.The soul dwells within us, a flame the size of a thumb. When it is known as the Lord of the past and the future, then ceases all fear:This in truth is That.Like a flame without smoke, the size of a thumb, is the soul; the Lord of the past and the future, the same both today and tomorrow:This in truth is That.As water raining on a mountain-ridge runs down the rocks on all sides, so the man who sees variety of things runs after them on all sides. But as pure water raining on pure water becomes one and the same, so becomes, O Nachiketas, the soul of the sage who knows.
This excerpt, like the one in my previous blog, comes from the translation of the Upanishads by Juan Mascaro.