What is the difference between Satan and the carnal soul? An Islamic view:

Man’s real identity which is called soul has numerous aspects and layers. Quran mentions its three states (carnal, self-reproaching and peaceful).
The carnal soul is formed when animalistic desires dominate man. It is a state of human soul that constantly commands human toward vices and to satisfy carnal desires.
But Satan is literally and terminologically applied to any untamed and defiant creature whether human, jinn or animal.
 Iblīs is a specific Satan that is a jinn upgraded to angels’ level because of long years of worship. However he defied God’s command (to prostrate Adam) and thus he was expelled from God’s compassion.  Iblīs vowed to misguide humans through temptation.
Therefore the carnal soul is in fact one of Satan’s tools to dominate and control humans. It is considered one of Satan’s soldiers.
Thus the temptation by Iblīs as the external Satan and temptation and stimulation by the carnal soul as the internal Satan would lead humans to collapse. In other words, in view of animal inclinations of humans, the carnal soul is influenced by Satan’s temptations giving way to Satan to advance step by step until the person is considered a member of Satan’s party.

The answer to this question requires a few premises:
First premise: The soul and its levels
Mankind’s real identity has three aspects and layers (animal, human and divine). It is well inferred from Quranic verses that human soul and spirit has three stages:
1-The carnal soul or man’s animalistic layer:
Man’s animalistic layer is summarized in lust, anger and carnal desires. This internal inclination and state of soul is termed by Quran as the carnal soul. The Quran emphasizes that: “Most surely (man’s) self is wont to command (him to do) evil” Quran 12:53
That is why it has been called the carnal soul (the soul that commands to the vices). In this stage the intellect and the faith have not become so strong to control and harness the untamed soul. Instead the intellect and faith become dominated and defeated by the carnal soul in many cases.
In a quotation by ancient Egypt’s queen, this stage has been referred to where she said: “And I do not declare myself free, most surely (man’s) self is wont to command (him to do) evil”. Quran 12:53

2-Self-reproaching soul:
“Lawwama soul”: It is a stage of soul reached by humans following education, training and effort. During this stage, human may occasionally commit vices due to the swelling of instincts. However he becomes immediately remorseful, blames himself and decides to recompense by cleaning his heart and soul through tawba [repentance].
Quran calls this stage “lawwama soul” and states: “Nay! I swear by the self-accusing soul.”Quran 75:2

3-Peaceful soul:
“Mutma’enna soul” is a stage achieved by humans following purification and complete training. In this stage untamed instincts are unable to fight with the intellect and faith because the intellect and faith have become so strong that instincts do not have much power to confront.
This is the position of prophets’ and God’s friends and their true followers; those who have learned lessons of faith and piety purifying their souls for many years  accomplishing jihad akbar [the grand jihad].
Quran calls this stage as “mutma’enna soul” and says: ” O soul that art at rest! Return to your Lord, well-pleased (with him), well-pleasing (Him)” Quran 89:27-28

Second premise:  Iblīs and Satan

1- Iblīs:  Iblīs is a specific Satan, a jinn upgraded to angels’ level due to long years of worship. However he was expelled from the heavens following his defiance because he disobeyed the divine command and committed a vice.

2-Satan: Satan is derived from the Arabic verb “sha-ta-na” meaning defiance and distancing. Any defiant creature is called Satan whether a human, a jinn or an animal.
As we read in Quran: “And thus did We make for every prophet an enemy, the Shaitans from among men and jinn.” Quran 6:112

 Iblīs is called Satan because he is a defiant, saboteur and untamed creature.

Third premise: The relation between the carnal soul and Satan
The carnal soul is in fact one of Satan’s tools to dominate and control humans. It is considered a soldier of Satan.
Thus the temptation by Iblīs as the external Satan and temptation and stimulation by the carnal soul as the internal Satan would lead humans to collapse.
Satan’s primary effort is to mislead humans hindering them from reaching the reality. He has sworn by divine might in this regard, as we read in Quran:” He [ Iblīs] said: Then by Thy Might I will surely make them live an evil life, all Except Thy servants from among them, the purified ones.” Quran 38:82-83
Eghwa is derived from the Arabic word “ghayy” meaning “against rushd” and “rushd” means to achieve the reality.
Satan has sworn by divine might that he will mislead humans -Quran 38:82-83 . He advances step by step to mislead humans influencing them by his temptations up to the point where humans become satans themselves. Then Satan employs them to mislead other human beings.
A human being who surrenders himself to his animalistic desires as a result of Satanic temptation, has in fact been entangled by the carnal soul.
Imam Ali (PBUH) says: “Like a hypocrite, the carnal soul flatters the person depicting itself as a friend so that it can control the human leading him to next levels”
Satan tempts people with weak faith and makes their hearts his house using their carnal desires and the carnal soul. Finally he touches their hands and bodies, shakes their hands and becomes their friend. Because one whose heart has become Satan’s house, he is not only Satan’s host but also his assistant. About this group, Imam Ali (PBUH) says: “…Thus Satan looks through their eyes and speaks through their tongues.”
Conclusion:
Satan and the carnal soul are both mans’ enemies.
Therefore Quran mentions Satan as man’s external enemy and advises him to consider Satan as his enemy. Quran 2:168
Also in traditions by the infallibles (Peace be upon them all) the carnal soul (whims) has been considered an enemy. That the holy prophet (PBUH) said: “Your worst enemy is your own soul” refers to the abovementioned stage of soul.
The secret to call the soul “the worst enemy” is because it is internal. The external enemy/burglar is unable to damage the person without the company of the internal enemy/burglar. This internal enemy is an insider who knows everywhere and applies this information to report the person’s desire to Satan and reports back Satan’s message (who is the external carnal soul). Therefore the carnal soul is considered one of Satan’s jonoud (soldiers) because many of the carnal soul’s characteristics are similar to those of Satan’s soldiers.
Therefore the carnal soul is influenced by Satan’s temptations giving way to Satan to advance step by step until the person is considered a member of Satan’s party.

Iblis (top right on the picture) refuses to prostrate before the newly created Adam from a Persian miniature

In Islam, the principle of evil is expressed by two terms referring to the same entity:Shaitan (meaning astray, distant or devil) and Iblis. Iblis is the proper name of the devil representing the characteristics of evil. Iblis is mentioned in the Quranic narrative about the creation of humanity. When God created Adam, he ordered the angels to prostrate themselves before him. All did, but Iblis refused and claimed to be superior to Adam out of pride. ^{[Quran 7:12]} Therefore, pride but also envy became a sign of “unbelief” in Islam. Thereafter Iblis was condemned to Hell, but God granted him a request to lead humanity astray, knowing the righteous will resist Iblis’ attempts to misguide them. In Islam, both good and evil are ultimately created by God. But since God’s will is good, the evil in the world must be part of God’s plan. Actually, God allowed the devil to seduce humanity. Evil and suffering are regarded as a test or a chance to proof confidence in God. Some philosophers and mystics emphasized Iblis himself as a role model of confidence in God, because God ordered the angels to prostrate themselves, Iblis was forced to choose between God’s command and God’s will (not to praise someone else than God). He successfully passed the test, yet his disobedience caused his punishment and therefore suffering. However, he stays patient and is rewarded in the end^]

Muslims hold that the pre-Islamic jinn, tutelary deities, became subject under Islam to the judgment of God, and that those who did not submit to the law of God are devils.

Although Iblis is often compared to the devil in Christian theology, Islam rejects the idea that Satan is an opponent of God and the implied struggle between God and the devil.^] Iblis might either be regarded as the most monotheistic or the greatest sinner, but remains only a creature of God. Iblis did not become an unbeliever due to his disobedience, but because of attributing injustice to God; that is, by asserting that the command to prostrate himself before Adam was inappropriate. There is no sign of angelic revolt in the Quran and no mention of Iblis trying to take God’s throne and Iblis’s sin could be forgiven at anytime by God. According to the Quran, Iblis’s disobedience was due to his disdain for humanity, a narrative already occurring in early New Testament apocrypha.

As in Christianity, Iblis was once a pious creature of God but later cast out of Heaven due to his pride. However, to maintain God’s absolute sovereignty, Islam matches the line taken by Irenaeus instead of the later Christian consensus that the devil did not rebel against God but against humanity. Further, although Iblis is generally regarded as a real bodily entity, he plays a less significant role as the personification of evil than in Christianity. Iblis is merely a tempter, notable for inciting humans into sin by whispering into humans minds (waswās), akin to the Jewish idea of the devil as yetzer hara.

On the other hand, Shaitan refers unilaterally to forces of evil, including the devil Iblis, then he causes mischief. Shaitan is also linked to humans psychological nature, appearing in dreams, causing anger or interrupting the mental preparation for prayer. Furthermore, the term Shaitan also refers to beings, who follow the evil suggestions of Iblis. Furthermore, the principle of Shaitan is in many ways a symbol of spiritual impurity, representing humans’ own deficits, in contrast to a “true Muslim“, who is free from anger, lust and other devilish desires.

In Sufism and mysticism

See also: Nafs

In contrast to Occidental philosophy, the Sufi idea of seeing “Many as One”, and considering the creation in its essence as the Absolute, leads to the idea of the dissolution of any dualism between the ego substance and the “external” substantial objects. The rebellion against God, mentioned in the Quran, takes place on the level of the psyche, that must be trained and disciplined for its union with the spirit that is pure. Since psyche drives the body, flesh is not the obstacle to humans but rather an unawareness that allows the impulsive forces to cause rebellion against God on the level of the psyche. Yet it is not a dualism between body, psyche and spirit, since the spirit embraces both psyche and corporeal aspects of humanity. Since the world is held to be the mirror in which God’s attributes are reflected, participation in worldly affairs is not necessarily seen as opposed to God. The devil activates the selfish desires of the psyche, leading the human astray from the Divine. Thus it is the I that is regarded as evil, and both Iblis and Pharao are present as symbols for uttering “I” in ones own behavior. Therefore it is recommended to use the term I as little as possible. It is only God who has the right to say “I”, since it is only God who is self-subsistent. Uttering “I” is therefore a way to compare oneself to God, regarded as shirk.

Read alsoThe Meaning of Nafs and the Struggle Against the Lower Soul ( JIHAD AL-NAFS)