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]:
1-The carnal soul or man's animalistic layer:
Man's animalistic layer is summarized in lust, anger and carnal desires[2]. 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" [3]
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[4] 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". [5]
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." [6]
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)" [7]
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.[8]
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.[9]
As we read in Quran: "And thus did We make for every prophet an enemy, the Shaitans from among men and jinn." [10]
 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." [11]
Eghwa is derived from the Arabic word "ghayy" meaning "against rushd" and "rushd" means to achieve the reality.[12]
Satan has sworn by divine might that he will mislead humans[13]. 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"[14]
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.[15]
About this group, Imam Ali (PBUH) says: "…Thus Satan looks through their eyes and speaks through their tongues."[16]
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.[17]
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" [18] 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.[19]
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.[20]

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