Part 13 · Chapter 159

And the matter is that although we explained that the principal [holder] of this name, "Ancient Holy One" (Atika Kadisha), is the Ein Sof that is clothed within the Atik, and therefore that supernal head which is not known, called Atik Yomin, is properly fit for this name "Ancient Holy One" in itself — unlike the two other heads of Arich Anpin. Nevertheless, it has already been explained there that they are all also called "Ancient Holy One."