The texts
The Upanishads are the oldest sustained inquiry into the Absolute that anyone wrote down. The earliest of them were composed in India before the Buddha taught, which puts them roughly twenty seven centuries behind us, and they arrive not as commandments handed down from on high but as conversations. A father teaches his son. A king questions a sage. A husband tells his wife why anything is loved at all. The tradition they founded is called Vedanta, the end of the Veda, because they stand at the close of India's oldest scriptures, and because after them there was nowhere further in to go.
The word
Brahman comes from a root that means to grow, to swell, to expand. It is not the name of a god. The Upanishads knew plenty of gods and set Brahman behind all of them, the way the ocean stands behind every wave. It is what everything rises out of, moves in, and settles back into, and the texts are blunt that no description reaches it. Whatever you can point to depends on something else, and Brahman is what all of it depends on.
That which is, one only, without a second.
That line is from the Chandogya Upanishad, and it is as close to a positive definition as the texts will come. Their more famous formula is a refusal, two words a teacher would repeat as the student reached for one wrong handle after another: neti neti, not this, not this.
Thou art that
The claim that made these texts immortal comes in the sixth chapter of the Chandogya, in a conversation between a father, Uddalaka, and his son Shvetaketu, home from twelve years of schooling and pleased with himself. The father has him drop a lump of salt in water and come back in the morning. The salt is gone. Taste the surface, the father says. Salt. The middle. Salt. The bottom. Salt. You cannot see it or grasp it, but there is nowhere in the water it is not. Like that, he says, is the subtle essence of all things. Then the sentence he repeats nine times, once at the end of each round of the teaching: tat tvam asi, thou art that.
It is one thing to say the Absolute exists. The Upanishads say the one searching for it is it. The ground of the world and the ground of the self are one ground. Twenty centuries later a Dominican friar preached the same identity from a Christian pulpit and was put on trial for it. The Chandogya says it nine times, calmly, as a father to a son.
Not this, not this
The method matches the map, and it is the method this site's front page calls subtraction. If Brahman were an object it could be studied, but it is the one thing never in front of you, because it is what is looking.
Thou couldst not see the seer of sight.
That is the sage Yajnavalkya in the Brihadaranyaka Upanishad, the oldest and deepest of the texts. His teaching was a peeling: not the body, not the breath, not the mind, not this, not this. What remains when everything that can be dropped has been dropped was his answer to every question that mattered, and he warned that it would never be an answer you could hold up and show anyone.
The same ground
The traditions gathered on this site differ in a thousand real ways, and honest reading does not flatten them. But set the Chandogya's subtle essence beside Eckhart's ground of the soul, the Tao that cannot be named, and the One of Plotinus, and the family resemblance is not manufactured. These are the furthest reports anyone has sent back from the inward direction, and at the far end they agree: the Absolute is not elsewhere, and you were never outside it.
Where to begin
Start with the sixth chapter of the Chandogya, for the salt and the father and the nine repetitions. Then the Brihadaranyaka, for Yajnavalkya. The translations made in the 1880s are long out of copyright and free everywhere online, and though their English is stiff in places, the conversations shine through. Read them the way they were composed: out loud, slowly, as if someone you trusted were telling you the one thing they knew.