Ein Sof, "without end", is the kabbalistic name for God as God is in himself, prior to all revelation, attribute and relation. The term emerges among the Provencal and Geronese kabbalists of the late twelfth and thirteenth centuries, in the circle of Isaac the Blind, and becomes standard in the Zohar.
Its whole purpose is to establish a distinction. On one side is the hidden Godhead, of which nothing whatever may be predicated: not will, not thought, not goodness, and in the sharper formulations not even existence in any sense in which the word is otherwise used. On the other side are the ten sefirot, the emanated attributes through which God becomes knowable, acts in the world, and is addressed in prayer. On this reading, everything scripture says of God is said of the sefirot; Ein Sof is the subject of no biblical sentence, and the negative theology is pressed further here than in most of the philosophical tradition.
That distinction generated the central problem of kabbalistic theology: what the sefirot are in relation to Ein Sof. If they are the divine essence itself, divine unity is preserved but God appears to have real internal multiplicity. If they are vessels or instruments distinct from the essence, simplicity is preserved but something has been placed between God and the world. Both positions had learned defenders, and the argument continued for centuries. Critics pressed the obvious charge: the fifteenth-century writer Isaac ben Sheshet transmits the barb that Christians believe in three and the kabbalists in ten. Kabbalists answered that the sefirot are not beings alongside God but the one God's self-articulation, related to Ein Sof as a flame to its colours, an analogy the Zohar itself supplies.