What Is Tazkiyah of the Nafs?
In Surah Ash-Shams, Allah has made seven oaths and then announced that success in the Hereafter depends on the purification of the self (tazkiyah of the nafs). Allah says:
وَ نَفْسٍ وَّ مَا سَوّٰىهَا۪ۙ (۷) فَاَلْهَمَهَا فُجُوْرَهَا وَ تَقْوٰىهَا۪ۙ (۸) قَدْ اَفْلَحَ مَنْ زَكّٰىهَا۪ۙ (۹) وَ قَدْ خَابَ مَنْ دَسّٰىهَاؕ (۱۰)
and by the soul, and the One who made it well. then inspired it with its (instincts of) evil and piety. Success is really attained by him who purifies it, and failure is really suffered by him who pollutes it.
"Inspired it with its wickedness (fujur) and righteousness (taqwa)" means that Allah has opened both paths, good and evil, in front of every person. Success (the good news of success in the Hereafter) is for the one who purifies their nafs (so that they can walk on the path of good). And for the one who does not purify it, the path of evil opens up, and the result is ruin in the Hereafter.
So what is tazkiyah of the nafs? To understand it, we need to reflect on the opening verses of Surah Al-Mu'minun.
قَدْ اَفْلَحَ الْمُؤْمِنُوْنَۙ (۱) الَّذِیْنَ هُمْ فِیْ صَلَاتِهِمْ خٰشِعُوْنَۙ (۲) وَ الَّذِیْنَ هُمْ عَنِ اللَّغْوِ مُعْرِضُوْنَۙ (۳) وَ الَّذِیْنَ هُمْ لِلزَّكٰوةِ فٰعِلُوْنَۙ (۴) وَ الَّذِیْنَ هُمْ لِفُرُوْجِهِمْ حٰفِظُوْنَۙ (۵) اِلَّا عَلٰۤى اَزْوَاجِهِمْ اَوْ مَا مَلَكَتْ اَیْمَانُهُمْ فَاِنَّهُمْ غَیْرُ مَلُوْمِیْنَۚ (۶) فَمَنِ ابْتَغٰى وَرَآءَ ذٰلِكَ فَاُولٰٓئِكَ هُمُ الْعٰدُوْنَۚ (۷) وَ الَّذِیْنَ هُمْ لِاَمٰنٰتِهِمْ وَ عَهْدِهِمْ رٰعُوْنَۙ (۸) وَ الَّذِیْنَ هُمْ عَلٰى صَلَوٰتِهِمْ یُحَافِظُوْنَۘ (۹)
Success is really attained by the believers. Who concentrate their attention in humbleness when offering Salāh (prayers). And who keep themselves away from vain things. and who are performers of Zakāh. And who guard their private parts. except from their wives or from those (bondwomen who are) owned by their hands, as they are not to be blamed. However, those who seek (sexual pleasure) beyond that are the transgressors. - and (success is attained) by those who honestly look after their trusts and covenant. and who consistently observe their prayers.
This blessed Surah begins with the words "The believers have certainly succeeded", and then describes the qualities of these believers. And in Surah Ash-Shams, success was made dependent on tazkiyah of the nafs. Putting the two together, it becomes clear that tazkiyah of the nafs (that is, success) belongs to those believers who have these qualities:
Keeping full watch over their prayers, with khushu in them
Avoiding even idle things (committing outward or inward sins is out of the question)
Acting upon Zakat
Guarding their private parts and limiting intimate relations to their lawful spouse
Honouring their trusts and promises
The system of training in Tasawwuf has been designed to attain exactly these qualities. Apart from this, some things fall within the scope of purifying the heart (qalb) and the intellect (aql), which are explained in the relevant section.