Yichud and perud, unity and division, can be said to be the pivotal concepts of Jewish mysticism.

Yichud is at the core of everything. All being, the whole of the creation, is as one body, the numerous members of which are fully interrelated and interdependent. However, just as in the analogy to the human body, the various organs and members are bound up one in the other yet each of them retains its own unique character and quality. Problems for the whole, or for the part, arise where this dual nature is ignored: when the particular shirks his universality, his membership in, and responsibility to, the others, the whole, and is preoccupied with himself. He commits an act of perud - division: mutilating the universe, 'cutting down the shoots.'

Perud - separation from the whole, in the mystic's view, is the cardinal sin, the very root of all sins. Separation is caused by self-assertion, ego-centricity. It is tantamount to idolatry, creating dualism or pluralism. It is an infringement upon the ultimate yichud -the unity or oneness of the Absolute. For to take that stance is to establish oneself as a yesh - a `something,' selfhood, a reality separate from, outside of, and next to, whatever other being there is. Thus it is a denial of the solitary unity of G‑d.

The consequences of this tragic separation and division are not limited to the offending individual. The severance of a part from the whole implies not only the rejection of the whole by the part, but also the loss of the part to the whole. The whole body is rendered incomplete, deficient. It has become incapacitated with regards to the unique qualities and functions of that member. Hence the mystic's emphasis on bitul hayesh, the duty to negate, to efface, the ego qua ego, and the imperative dissolution in the whole, the concept of devekut - to strive for the unio mystica.