Tanāsukh (transmigration of souls) in bodies, if it occurs within the same species, is muḥāl (impossible). In this ḥāl (state), every temperament would have to possess a nafs (the lower self) transferred from another. If another nafs were to unite with it, then an animal would possess two nafs. Furthermore, it is not a rule that every mortal must unite with a matter, nor that the number of united nafs must equal the number of matters. Nor is it a rule that the number of things united with nafs and those separate from them must be equal. When a body is given to these nafs, it either unites with it or resists and withdraws. Then this expands. In other contexts, it suffices with what it finds pertaining to us. (Ishārāt, IV, 37)
The meaning of tanāsukh is the passage of the nafs from one body to another body. The Ahl al-Sunnah (mainstream Sunni scholars) have rejected this, while the extreme Rāfiḍīs have accepted it. The major philosophers have also not accepted it. Among those who accept tanāsukh, there are differing views. Some consider it possible, while others deem it muṭlaq (absolutely) necessary. They have also disagreed on other points.
Some of them say: The form of the species must be preserved in individuals. The nafs only passes from one human to another human body. According to them, this type of transfer is called naskh. Others argue that it is not necessary to preserve the species in individuals, and that the transfer can occur from humans to other beings and animals. This is called ta'nīs (feminization).
They call the transfer from animal to plant mesh. Some consider transfer from animal to plant permissible; this is called faskh. There are also those who accept transfer to other inanimate substances, which they call resh.
Those who deem preservation of the species' form necessary have said: If the one to be transferred is harmful and ignorant, it passes into a lower body.
They have also said: The nafs continues to pass from one body to another until it is purified and perfected from all corporeal attachments. Only then does it attain the realm of holiness and a stable purity.
Those who accept the transfer of the nafs to animals say: This is a punishment for it, for the nafs is then in deep darkness and shiddat (intensity). All these are great imbalances, baseless absurdities without evidence or support. Rather, this is to exercise taḥakkum (arbitrary authority) over Allah regarding His servants and to fabricate lies about Allah's unseen. Especially, this is to report a realized matter that requires definitive proof, for which conjecture is insufficient. Matters of knowledge related to obligations are not like this; in those, conjecture suffices. (Rawḍa, 210)