Meaning of nafs: It has two meanings. First, it means the powers of anger and lust in a human being ... and this is the usage mostly found among the [so-called] people of tasawwuf, who take nafs as the comprehensive word for all the evil attributes of a person. That is why they say: one must certainly do battle with the soul/self and break it (la budda min mujahadat al-nafs wa kasriha), as is referred to in the hadith: A'da 'aduwwuka nafsuka al-lati bayna janibayk (Your worst enemy is your nafs which lies between your flanks). Al-'Iraqi says it is in Bayhaqi on the authority of Ibn 'Abbas and its chain of transmission contains Muhammad ibn Abdur-Rahman ibn Ghazwan, one of the forgers.
The second meaning of nafs is the spirit, the human being in reality, his self and his person. However, it is described differently according to its different states. If it assumes calmness under command and has removed from itself the disturbance caused by the onslaught of passion, it is called "the satisfied soul" (al-nafs al-mutma'inna) ... In its first meaning the nafs does not envisage its return to God because it has kept itself far from Him: such a nafs is from the party of Shaytan. However, when it does not achieve calmness, yet sets itself against the love of passions and objects to it, it is called "the self-accusing soul" (al-nafs al-lawwama), because it rebukes its owner for his neglect in the worship of his master ... If it gives up all protest and surrenders itself in total obedience to the call of passions and shaytan, it is named "the soul that enjoins evil" (al-nafs al-ammara bi al-su') ... which could be taken to refer to the soul/self in its first meaning. [Definitions at the beginning of the book Kitab Sharh 'Aja'ib al-Qalb (Book of the Explanation of the Mysteries of the Heart)]
God has armed soldiers which He has placed in the hearts and the souls and others of His worlds, and none knows their true nature and actual number except He ... [He proceeds to explain that the limbs of the body, the five senses, will, instinct, and the emotive and intellective powers are among those soldiers] Know that the two soldiers of anger and sexual passion can be guided by the heart completely ... or on the other hand disobey and rebel against it completely, until they enslave it. Therein lies the death of the heart and the termination of its journey towards eternal happiness. The heart has other soldiers: knowledge ('ilm), wisdom (hikmah) and reflection (tafakkur) whose help it seeks by right, for they are the Party of God against the other two who belong to the party of Shaytan ...
God says: "Have you seen the one who takes as his god his own desire?" [Al-Qur'an 25:43] And "He followed his own desire. So his example is like that of a dog: if you chase him he pants, or if you leave him, he [still] pants." [Al-Qur'an 7:176] And about the person who controlled the passion of his soul/self God says: "But as for he who feared the standing before his Lord and restrained the soul from [his] desire, then indeed, Paradise will be his refuge." [Al-Qur'an 79:40-41]
Know that the body is like a town and the intellect of the mature human being is like a king ruling that town. All the forces of the external and internal senses he can muster are like his soldiers and his aides. The soul/self that enjoins evil (nafs ammara), that is, lust and anger, is like an enemy that challenges him in his kingdom and strives to slaughter his people. The body thus becomes like a garrison-town or sea-outpost, and the soul like its custodian posted in it. If he fights against his enemies and defeats them and compels them to do what he likes, he will be praised when he returns to God's presence, as God said: " ... [who strive and fight] in the way of God with their wealth and their lives over those of remained [behind], by degrees. And to all God has promised the best [reward]." [Al-Qur'an 4:95] [Section entitled: "The Soldiers of the Heart" in the same book]
c) The thoughts that stir one's desire are of two kinds ... praiseworthy, and that is called "intuition" (ilham), and blameworthy, and that is called "whispering" (waswasa) ... The heart is owned mutually by a shaytan and an angel ... The angel stands for a creature which God has created for the overflowing of benefit, the bestowal of knowledge, the unveiling of truth, the promise of reward, and the ordering of the good ... The shaytan stands for a creature whose business is to be against all this ... Waswasa against ilham, shaytan against angel, success (tawfiq) against disappointment (khidhlan).
The Prophet sallallahu 'alayhi wa sallam said: "There are two impulses in the soul, one from an angel which calls towards good and confirms truth; whoever finds this let him know it is from God and praise Him. Another impulse comes from the enemy which leads to doubt and denies truth and forbids good; whoever finds this, let him seek refuge in God from the accursed devil." Then he recited the verse: "Satan threatens you with poverty and orders you to fahshah (immorality)." [Al-Qur'an 2:268] [At-Tirmidhi: hasan; Nasa'i; 'Iraqi did not weaken it].
Al-Hasan al-Basri said: "Two thoughts roam over the soul, one from God, one from the enemy. God shows mercy on a servant who settles at the thought that comes from Him. He embraces the thought that comes from God, while he fights against the one from his enemy." To illustrate the heart's mutual attraction between these two powers the Prophet sallallahu 'alayhi wa sallam said: "The heart of a believer lies between two fingers of the Merciful." [Muslim, Ahmad, At-Tirmidhi, Ibn Majah] ... The fingers stand for upheaval and hesitation in the heart ... If man follows the dictates of anger and lust, the dominion of Shaytan appears in him through idle desires (hawa) and his heart becomes the nesting-place and container of Shaytan, who feeds on hawa. If he does battle with his desires and does not let them dominate his nafs, imitating in this the character of the angels, at that time his heart becomes the resting-place of angels and they alight upon it ...
The Prophet sallallahu 'alayhi wa sallam said: "There is none among you in whom there is not a devil." They said: "Even in you, O Messenger of God?!" He said: "Even in me, but God helped me to overcome him and he has submitted to me, so he doesn't order anything except good." [Muslim] ... The mutual repelling of the soldiers of the angels and the devils is constant in the battle over the heart, until the heart is conquered by one of the two sides which sets up its nation and settles there ... And most hearts have been seized by the soldiers of Shaytan, who fill them with the whispers that call one to love this passing world and disregard the next. [Section entitled: "Shaytan's domination over the heart through whispering (al-waswasa)" in the same book]
d) The Prophet sallallahu 'alayhi wa sallam said: al-mujahidu man jahada nafsahu fi ta'at Allah 'azza wa jall ("The struggler is the one who strives against his soul/self in obedience to God, the Mighty and Majestic.") [At-Tirmidhi, Ibn Majah, Ibn Hibban, Tabarani, Hakim, etc.] ... Sufyan ath-Thawri said: "I never dealt with anything stronger against me than my own soul/self; it was one time with me, and one time against me" ... Yahya ibn Mu' adh ar-Razi said: "Fight against your soul/self with the four swords of training: eat little, sleep little, speak little, and be patient when people harm you ... Then the soul/self will walk the paths of obedience, like a fleeing horseman in the field of battle." [Section entitled: "Proofs..." from the book Kitab Riyadat al-Nafs wa Tahdhib al-Akhlaq wa Mu'alajat Amrad al-Qalb (Book of the training of the soul/self and the disciplining of morals/character and curing thee disease of the heart)]
From Ihya 'Ulum ad-Din (The Revival of the Religious Sciences)