“But it is not only the good that has its origin in the irradiation of lights of the emanation; there also exists a positive evil, which is related to the root of death. In this way Isaac adopts a modified form of the corresponding idea of the Bahir concerning the nature of evil.
The oppositions discussed in chapter 4 of the Book Yesirah, in connection with the seven consonants of the Hebrew alphabet that can be pronounced in two different ways, also include that of life and death; and Isaac explains that “after the cause of life, the cause of death was emanated.”
Also at the beginning of chapter 2 he emphasizes that in each case the poles of oppositions come “from an autonomous principle.” Death is therefore something more and something other than the mere cessation of life. It has a positive root.
Certainly Isaac stresses at the same time that in the world of the sefiroth and of the “inner essences,” good and bad are not yet dissociated from one another but are harmoniously united, be-‘ah-duth ube-shalom. Only when the “roots” continue to develop into a tree—and in the emanation that later derives from it and to which the double letters correspond—does evil also exist in isolation.
Otherwise, Isaac’s thinking remains determined by the image of the organic body (on chap. 4). The letters are like the branches of the tree and in them the intertwined pattern of good and evil is unfolded, so that each good middah also has its corresponding evil and vice versa.”
Gershom Scholem, Origins of the Kabbalah, p. 293.