For the kabbalists [the medieval mystics] the existence of evil was a central issue. According to one tradition evil has no objective reality — human beings are unable to receive all of the influx from the sefirot [the emanations/attributes of God], and it is this inability which is the origin of evil. Created beings are therefore estranged from the source of emanation and this results in the illusion that evil exists.
Another view, as propounded in the Bahir [the earliest kabbalistic work], depicts the sefirah of power as an attribute “whose name is evil.” On the basis of such a teaching Isaac the Blind [13th century] concluded that there must be a positive root of evil and death [i.e. evil does not arise from the inability of humans to receive the emanations of the sefirot]. Following this view, most kabbalists believed that evil is “the other side” (Sitra Ahra) which is opposed to divine abundance and grace. According to some kabbalists, the Sitra Ahra should be conceived of as counter‑sefirot, a realm of dark, unclean powers opposed to holiness and goodness. Thus the Zohar [the most important kabbalistic text] states:
At the beginning of the night, when darkness falls, all the evil spirits and powers scatter abroad and roam about the world, and the ‘other side’ sets forth and inquires the way to the King from all the holy sides. As soon as the ‘other side’ is roused to this activity here below, all human beings experience a foretaste of death in the midst of their sleep. As soon as the impure power separates itself from the realm above and descends to begin its rule here below, three groups of angels are formed who praise the Holy One in three night watches, one following another, as the companions have pointed out. But whilst these sing hymns of praise to the Holy One, the ‘other side,’ as we have said, roam about here below, even into the uttermost parts of the earth. Until the ‘other side’ has thus departed from the upper sphere, the angels of light cannot unite themselves with their Lord.
For these kabbalists the evil forces are engaged in a battle with the powers of holiness. These evil powers came into being through the supra‑abundant growth of the sefirah of Judgment (Din) when it separated from the sefirah of Compassion (Rahamim). This realm is continually strengthened through human sin.
In an interpretation of the Garden of Eden, Adam’s eating of the Tree of Knowledge of Good and Evil is understood as an allegory of the first appearance of evil. Here Adam is understood as separating the Tree of Knowledge from its fruits, thereby activating the potential evil contained within the Tree by bringing about a division in the divine unity. Thus the channels between the upper and lower worlds were unsettled: the lowest sefirah Kingdom (Malkhut) was separated from the others, and the unity between Creator and creation was divided. Other sins in the history of ancient Israel were also interpreted in a similar light—in each case divine unity was disturbed, and it is only through the good deeds of biblical heroes that cosmic repair can take place.
According to the Zohar, evil is like the bark of a tree of emanation: it is a husk or shell in which lower dimensions of existing things are encased. As the Zohar relates:
King Solomon, when he penetrated into the depths of the nut garden took a nut shell (kelippah) and drew an analogy from its layers to these spirits which inspire sensual desires in human beings, as it is written, ‘and the delights of the sons of men (are) male and female demons’ (Ecclesiastes 2:8). This verse also indicates that the pleasures in which men indulge in the time of sleep give birth to multitudes of demons. The Holy One, blessed be He, found it necessary to create all these things in the world to ensure its permanence, so that there should be, as it were, a brain with many membranes encircling it. The whole world is constructed on this principle upper and lower, from the first mystic point up to the furthest removed of all the stages. They are all coverings one to another, brain within brain, and spirit within spirit, so that one is a shell to another.
In this context evil is understood as a waste product of all organic process—it is compared to bad blood, foul water, dross after gold has been refined and the dregs of wine. Yet despite this depiction, the Zohar asserts that there is holiness even in the Sitra Ahra regardless of whether it is conceived as a result of the emanation of the last sefirah or as a consequence of man’s sin. The domains of good and evil are intermingled, and it is man’s duty to separate them.