The rationale behind reincarnation or transmigration is dealt with in the Zohar in a long passage called Saba d'Mishpatim, a portion of which has been translated below. The central idea is that reincarnation, or gilgul, has two purposes: a) to rectify sin; b) to acquire higher levels of soul. The former types of souls are the "old souls" referred to above, while the latter are "new souls," which do not require rectification as such. The following section is based on the verse, "If you buy a Jewish slave, he shall work for six years and in the seventh year he shall go free, without liability." (Ex. 21:2) A Jewish person can become a slave in one of two ways: a) Due to extreme poverty, he sells himself. (In this case the slave's master is obligated to treat him as a hired laborer rather than as a slave per se, providing for his needs and those of his entire family. (see Lev. 25:39) This is not the type of slave discussed below.) b) he is sold by the court after conviction as a thief, if he does not have the means to repay what he stole. This is the type of slave discussed in this section.
The soul must be reincarnated either because of sin or because it failed to completely fulfill its obligations in Torah and mitzvot
Rabbi Shimon bar Yochai began his discourse reciting the verse:
"These are the ordinances ('mishpatim') that you shall place before them… (Ex. 21:2) The Targum translates this as "These are the judgments ['mishpatim'] that you shall arrange before them." This refers to the coordination of gilgulim [reincarnations, sing. gilgul] - the judgments of souls that must be reincarnated and return to this world to receive the punishments to which they have been condemned.
"If you buy a Jewish slave, he shall work for six years and in the seventh year he shall go free, without liability." Rabbi Shimon said to his circle of disciples: Now is the time to reveal a number of closely-guarded secrets concerning gilgul: "If you buy a Jewish slave, he shall work for six years" - this applies when the soul must be reincarnated, either because of sin or because it failed to completely fulfill its obligations in Torah and mitzvot while alive in this world. Now if the person's soul is from the aspect of the angel Meta‑tron, comprising six levels chesed, gevura, tiferet, netzach, hod and yesod of the world of Beriya, then the verse states, "he shall work for six years." The soul is obligated to undergo gilgul only until it completes the rectification of those six levels, in regard to that aspect whence it was taken - the six sefirot of Meta‑tron just mentioned.
However, if the soul is from the aspect of the Shechina malchut of Atzilut, the seventh of the emotional sefirot, corresponding to Shabbat, the seventh day, what is written? "In the seventh year he shall go free, without liability," because a righteous tzadik who has merited a soul from malchut of Atzilut does not have any work in the sense of rectification. Accordingly, the gilgul for such a soul is not a rectification or punishment of any sort, but for another purpose - for the sake of benefiting the world and providing leadership. Such a soul is comparable to the Sabbath, the seventh day, when all types of work are prohibited. Indeed, elsewhere a Tzadik and a Torah scholar are called "Shabbat." And since he has no obligation to work, he is not enslaved. Regarding such a soul it states, "in the seventh year he shall go free, without liability." "Without liability" - without being enslaved.
Rabbi, what is unique about that soul regarding which it states, 'the seventh day is Sabbath to the Lord your God; you shall not do any work...'
Just then an old man [Saba, literally "grandfather" in Hebrew] came down to Rabbi Shimon bar Yochai, and said: If this is so, Rabbi, what is unique about that soul regarding which it states, "the seventh day is Sabbath to the L-rd your G‑d; you shall not do any work - you, your son, your daughter, your slave, your maidservant, your animal?" (Ex. 20:10) Since even "your slave and your maidservant" - souls from the world of Beriya and Yetzira - and your animals - souls from the world of Asiya- are prohibited from doing any work on the Sabbath, just like "you, your son and your daughter" - souls from Atzilut - what difference is there between them?
He [Rebbe Shimon] replied: Saba, Saba! This you are asking? This entire verse is certainly said regarding the soul of a righteous tzadik, which is from Atzilut. Although it is obligated to descend and be reincarnated in all these levels, even as a slave and a maidservant, and in animals, which are the ofanim, or in any beasts into which human souls transmigrate, nevertheless, the verse "you shall not do any work" is written in regard to it. When the soul of a tzadik from the world of Atzilut is obligated to descend into this world, and be reincarnated even into beasts of the forest, it nevertheless does not have to do any work [of rectification]. This is the meaning of "you shall not work him like a slave." (Lev. 25:39) In regard to a tzadik, who is the Sabbath day, "you shall not work him like a slave," who is the weekday.
The Sabbath…is the soul-mate of the tzadik…
But, Saba, Saba! The Sabbath, which is an only daughter, i.e., malchut of Atzilut, called bat (daughter), is the soul-mate of the tzadik , who is also called the Sabbath, as explained above. But since this is so, since she is his soul mate, why is it written in regard to him, "and if he shall take another in addition to her…?" (Ex. 21:10) If he has found his soul mate, why would he need an additional mate? He said: This is without a doubt the difference: "and if he shall take another" refers to the weekday aspect of the Sabbath. The meaning of this phrase will be explained shortly. For there are ordinary weekdays that are not called the weekday aspect of the Sabbath, but rather a spiritually impure slave girl (shifcha).
The old man asked: What is "the weekday aspect of the Sabbath?"
Rabbi Shimon replied: this is the amma [a maidservant] bina of malchut of the world of Beriya who is the body for the only daughter. The only daughter [malchut of the world of Atzilut] is clothed within an amma like the soul is clothed in the body. Regarding her it is said, "and if he shall take another in addition to her."
The explanation of this is as follows:
Come, look! There are souls called 'amma' and there are souls called 'shifcha' (slave-girl), and there are souls called 'the daughter of the king.' Each is named according to the source whence it is drawn, or to where it transmigrates. And in the verse there is also a man - "if a man will sell his daughter as a maidservant…" (Ex. 21:7) regarding whom it is said, "the L-rd is a man of war," (Ex. 15:3) referring to Zeir Anpin of Atzilut, and there is also another man, regarding whom it is said, "and the man Gabriel," (Daniel 9:21) referring to the archangel deriving from gevura of Beriya. This part of the verse is therefore interpreted as: "if a man [the Holy One blessed be He] will sell his daughter [a soul from Atzilut, which is called "an only daughter"] as a maidservant [the soul transmigrates to that level of Beriya called amma]…."
That is, the Jewish people will not leave the exile in the manner of slaves - hastily and in flight
Therefore, a soul that must be reincarnated, if she is a daughter of the Holy One blessed be He i.e., if the soul derives from malchut of Atzilut, do not say that she may be sold into the body of a gentile, Heaven forbid, for that is where the yetzer hara [evil inclination] from 'Satan' rules. As it is written, "I am G‑d, this [the Tetragrammaton] is My name; I will not give My glory to another" (Isaiah 42:8) - i.e., to the yetzer hara. Thus, if the soul is from Atzilut, even though it transmigrates to the world of Beriya, it is not obligated to do the work of rectification, nor is it subject to the kelipot.
Now regarding the body in which the daughter of the King [the soul from Atzilut] resides - do not say that she can be sold to the lower crowns of impurity, may Heaven have mercy upon us. In this regard the verse states, "the land [malchut] shall not be sold in perpetuity, for the land is Mine." (Lev.25:23) Now what is the body of the King's daughter? This is the angel Meta‑tron, who is the amma [maidservant] of the Shechina, since the Shechina is clothed within him. Thus the body becomes an amma [maidservant] for the King's daughter who is held captive therein, and it nevertheless transmigrates in gilgul. What is written in this regard? "If a man sells his daughter as an amma [maidservant] she shall not go free like the going free of a slave." (Ex. 21:7)
Furthermore, another interpretation of the verse: "If a man sells" refers to the Holy One blessed be He; "his daughter" refers to the Jewish people, who are the aspect of an only daughter malchut, which is called "daughter." Now if you will say that they will go free in the future, with the advent of the Mashiach, like those enslaved in Egypt, who were from the aspect of a slave, Meta‑tron, and fled from Egypt, the verse states: "you shall not depart in haste, nor in flight." (Isaiah 52:12) That is, the Jewish people will not leave the exile in the manner of slaves - hastily and in flight, in fear of their former slave-masters, because they will not be from the aspect of slaves, but rather like the only daughter of the Holy One blessed be He. Thus, when a person is worthy of the souls from the aspect of "the only daughter", it is said regarding this soul, "she shall not go free like the going free of a slave."
[Translation and commentary by Moshe Miller]