.‘Reincarnation’ means transmigration of one’s soul (Nafs) or other abstract beings (Mujarradat) in another body after death
 It should be mentioned that according to the believers in reincarnation, the manner of appearance of the spirit (Rouh) and soul (Nafs) into the body of another varies. They believe since the spirit of man does not attain perfection during this life, after it leaves the body (death), the spirit should transmigrate in another body for perfection.
 Some believers in reincarnation say: If the spirit of man moves to a body of another human, this reincarnation is called naskh. If man is reincarnated as an animal they call it maskh and if the spirit moves to a plant it is faskh and raskh is when movement to an inanimate thing is possible.
 As mentioned above some who believe in reincarnation explain the above four stages. These are complex stages and require further explanation.
 Although many scientists such as Pythagoras and Epicurus and some followers of the philosophy of Masha’ and others, whose names we do not find necessary to mention here, believe in reincarnation, we believe, based on the Holy Quran and narrations by our Imams, that any viewpoint which finds reincarnation true in any name and form is completely unacceptable.
 A significant viewpoint by one of the believers in reincarnation is that of a sage called Yuz Asaf. Mulla Sadra has written about him on page 8 of Volume 9 of his book, Asfar. Yuz Asaf says:
 I was living during Noah’s flood and warned my people.
 (He believes his soul was previously in the body of another and was traveling elsewhere.)
 This is the basis of Yuz Asaf’s viewpoint on reincarnation, which needs to be entirely investigated. However, we have only mentioned his narration here and will not presently give any evidence on rejection of the same as we are only discussing reincarnation.
 Another viewpoint of Yuz Asaf is:
 When the soul and abstract beings leave the body, the spirit of the righteous is connected to a divine world and attains such prosperity that no eye has ever witnessed, no ear heard and no heart felt this kind of magnificence in the air, land, tree, breeze, light or warmth in this material world.
 He says: After considering the spirits of the righteous, we get to those spirits whose deeds are midway between good and evil and to the evildoers who reincarnate in the body of animals and sometimes as plants or inanimate things.
 This was an example of Yuz Asaf’s viewpoint on reincarnation. You, as a researcher, should study these issues with an open mind.
 Concerning the first issue brought up by Yuz Asaf, which is regarding traveling of the souls of the righteous and connection to the divine world, this viewpoint has nothing to do with reincarnation. Many verses in the Holy Quran, narrations by our Imams and all divine schools have stated, with only minor differences, that the spirits of the righteous have such a station.
 With regard to the second issue, where he says the spirits of those whose deeds are midway between evil and good and the evildoers are reincarnated in bodies of animals, plants and inanimate things, this is confirming reincarnation, which is entirely invalid according to the Holy Quran and narrations of our Imams.
 One of the reasons given for the rejection of reincarnation is as follows:
 Why is reincarnation unacceptable?
Reply: The necessity for two souls coming together in one body is the cause for reincarnation being invalid and for the beliefs of the followers of reincarnation to be rejected.
 In other words, when the body of a human being has full capability for appearance of the soul (some consider this when the foetus is 4 months in the mother’s womb and some earlier) the Source of Blessings, grants him a suitable soul which has connection with that body. This means that at the moment a being is capable to have a face outside, God the Almighty grants his soul to him. Just like a dark house which appears before light and achieves light from it, the body too benefits from the light of a soul which it deserves.     
 Those who travel in the path of philosophy should be aware that the soul is part of jowhar mafaregh and its illumination shines upon the body. This means that the illumination of each person’s divine soul should be upon the same body and no other one.
 According to the doctrine of reincarnation, if another body is attributed to the same soul, it will be necessary for one body to have two controllers whereas the concept of two controllers over a single object at the same time is not possible and this is rejected.

Resource:  Part of researches by Esteemed Philosopher, Professor Mosavi on volume 9 of Mulla Sadra’s book, Asfar.