Excerpt from
The Transmigration of Souls (Part I Chapter 5)
Rabbi Isaac LURIA
A Secret to the Age and Number 13
"Soul impregnation ( Ibbur ) occurs when a soul enters the body of someone who is already born and has reached maturity.
When another soul enters such a person, the latter is, in a way, like someone carrying a baby inside her—hence the term "soul impregnation'"
As already said, this happens only in an adult, that is one who is at least 13 years and one day old, because from that age he is an adult and obligated to keep all the religious requirements. Therefore only then does that other soul enter him in order to help him and make him a righteous man by urging him to obey all the commandments of the Holy law. This soul impregnation takes place on two accounts:
Firstly if the incoming soul had been unable to perform a duty that was not of such a nature that it was compelled to' undergo a further metempsychosis (Gilgul), and therefore it enters the man simply for another opportunity to fulfill the obligation.
Or, secondly, if that soul is visiting the soul already present, because the owner of the latter needs the help of the additional soul to make him righteous and to guide him. For the, newly arrived soul is faultless. In either case, the soul impregnation does not occur before the age of 13 years and one day.
What is more, there is the following difference between the two cases: when the impregnating soul joins the original one in order to make good its own deficiencies, it diffuses itself, like the host's own soul, throughout his whole body, it suffers like the latter all the pains and miseries of the body and has to remain incarnated there until it has managed to fulfill all its outstanding obligations. It then leaves the person. But when the impregnating soul enters a person because he needs its support, it does not suffer any of the pains and miseries of his body, as the helping soul is not itself deficient and has not arrived on its own behalf. Hence there is no set time before which it may not leave the body, but it dwells in the body for a longer or shorter period at its own discretion. If the person does what is good, it stays with him and unites with him more intimately the better he becomes; but if he does what is evil, it separates from him on its own Authority."
Below is a link to one of the most profound and important Kabbalsitic texts ever written The Shaarey Kedushah (The Gates of Holiness)
The Shaarey Kedushah (The Gates of Holiness)