the word nafs (soul) is used to refer to the soul that follows its desires, or to its following those desires. By contrast, the word rūh (spirit) is not used in this sense, since it does not designate the soul in its capacity as the governor of the body.
Souls are said to be of three types:
The “soul that commands evil,” which is predominantly driven to follow its desires by committing sins and acts of disobedience.
The “self-reproaching soul,” which sins and repents. Both good and evil proceed from it, but when it does evil, it repents and turns back to Allah. It is called self-reproaching because it reproaches its possessor for sins, and because it wavers—that is, fluctuates between good and evil.
The “tranquil soul,” which loves and desires goodness and good deeds, and hates and abhors evil and bad deeds. This has become its character, its habit, and its settled disposition.
These are attributes and states of one and the same entity; each human being has only one soul. This is something a person recognizes within himself.
Some physicians who espouse philosophy have said that there are three souls: a vegetative soul located in the liver, an animal soul located in the heart, and a rational soul located in the brain. If they mean by this three faculties associated with these organs, that is conceded. But if they mean three distinct, self-subsisting entities, this is a manifest error.