The Powers of the Soul to Experience God
Introduction: The Internal Dimension
Yichud is the spiritual state associated with the sefirah of da'at.
Yichud describes the unification, or becoming one, of two bodies or souls. Upon the creation of Adam and Eve, it is said (with reference to all future mankind): "And he shall cling to his wife and they shall become one flesh" (Genesis 2:24). Subsequently, the union of Adam and Eve is described as an act of da'at: "And Adam knew his wife Eve" (Genesis 4:1).
The power of yichud (whether on the spiritual or physical plane) is thus seen to be rooted in one's power of da'at, one's ability to recognize his chosen one and establish with him/her an authentic rapport. Yichud is thus the process of "bringing back together" the two lost halves of what was initially a single entity.
All of the kavanot of Kabbalah are referred to as yichudim, for they all intend to effect unifications between the manifold dimensions of reality (beginning with those in the Divine realm of Atzilut itself). The intention that all one's deeds be "for the sake of heaven" is referred to as yichud hama'aseh ("the unification of deed"). (Also see Yabok).