So here, we must know that when an Adam (a person) merits a Nefesh, Ruach, and Neshama [all together the first time he is alive] and he later blemishes (damages) them, as we explained, when he comes back in a gilgul, all three cannot come together; only each one of them in a gilgul of its own. And here we must know what is the law regarding this Nefesh, Ruach, and Neshama. Also, know that when a Nefesh reincarnates into another body to rectify itself, and it [successfully] rectifies itself, the Ruach is unable to enter into that body, as we explained, because how can a blemished Ruach dress itself into a perfected Nefesh?
And if you would suggest that the blemished Ruach dresses itself into the blemished Nefesh before the Nefesh was perfected, this also is not possible, because a Ruach cannot enter until the rectification of the Nefesh is completed. This is because the Nefesh is at a lower level (than the Ruach), and so the Ruach is required to come into another gilgul on its own, e.g., resting on the Nefesh of a Ger [who has a new Nefesh], and there (in the Ger) the Ruach is rectified.
And the same thing happens with the Neshama - it can only be rectified in the same way -- resting on the Nefesh of a Ger. And this is the secret of what is written in בסבא דמשפטים, p.98:2, that blemished Neshamot [are rectified] in Geirim, and they are merited through them. Specifically, a Ruach alone, or a Neshama alone, cannot dress themselves (rest upon / enter into) the body of another; rather, they could only rest upon a Nefesh. So they rest upon the new Nefesh of the Ger, and through that they are rectified.