Know, that if a person merits obtaining his Nefesh, Ruach, and Neshama, and then blemishes them through sin, he will have to be reincarnated to rectify the damage.
The process of gilgul and tikun will be explained throughout the rest of the book, starting with the next chapter. The main point here is in this next paragraph.
When he returns in a gilgul with his Nefesh and he rectifies it, his Ruach will not join him. This is because his Ruach remains blemished, and it cannot rest upon a rectified Nefesh.
The Hebrew word for “convert” is ger, a word that also means “stranger.” Probably both meanings are applicable here.
In other words, rectified levels of souls do not reside in the same body with blemished ones. In what was discussed previously, the person was adding non-blemished, new divisions of soul to already rectified divisions of his own soul. However, once he has sinned and must come back another time, the process of tikun changes. He cannot add blemished aspects of soul on top of parts that have already been rectified.
Therefore, his [blemished] Ruach will be reincarnated into another person, joining up with the Nefesh of a convert. The Neshama will likewise do the same.
The Hebrew word for “convert” is ger, a word that also means “stranger.” Probably both meanings are applicable here. The Nefesh that will host this blemished Ruach must be the Nefesh of a convert, but relative to the homeless Ruach it is also the Nefesh of a stranger.
The reason for this is explained elsewhere, in chapter 4, section 2, based on Shaar Maamrei Rashbi, Parashat Mishpatim, 98.
Righteous people are greater in death than during theit lifetimes.
And the Nefesh that was rectified will receive a rectified Ruach of a righteous person who was similar to him in some of the particular good deeds that he performed. It will actually take the place of his own Ruach. Similarly, if he rectifies his [blemished] Ruach completely, then he will receive a Neshama from some righteous person, which will act in place of his own Neshama. This is the esoteric meaning of what Chazal say: “Righteous people are greater in death than during their lifetimes (Sanhedrin 47a).”
Since their Ruach can fulfill this important function in the lives of others.
Now, after this person dies, his [own rectified] Nefesh will go together with this Ruach [of a righteous person] and through it [i.e. the Ruach] receive the blessing fitting for itself. When his own Ruach, which joined with the Nefesh of a convert, becomes completely rectified, then his original Nefesh will say, “I will go and return to my first husband,” since it has been rectified.
In other words, after parts of his own soul are rectified, they can later be reunited in another gilgul, and return together.
It works in the same way for the Neshama with respect to the Ruach. After a person dies, they return in a gilgul and achieve tikun together.
End of [Original] Introduction (Chapter One).
[Commentary by Shabtai Teicher.]