Tazkiyah means purification of the self. It is the ongoing inward work of cleansing the heart of destructive qualities, and cultivating good ones: the inner counterpart to the outward conduct covered in the lesson on character.

The Qur'an ties this directly to a person's ultimate outcome, twice, in similar words. In Surah Ash-Shams, chapter ninety-one, an English translation of the meaning reads: “He has succeeded who purifies it, And he has failed who instills it with corruption.” And in Surah Al-A'la, chapter eighty-seven, verse fourteen: “He has certainly succeeded who purifies himself”.

Classical scholars describe a number of diseases of the heart to guard against. The first is pride. A hadith in Sahih Muslim describes it in unusually strong terms: “He who has in his heart the weight of a mustard seed of pride shall not enter Paradise”.

The second is destructive envy: wishing that a blessing were taken away from someone else, not simply wishing you had the same. A hadith in Sahih al-Bukhari, number fourteen hundred and nine, narrows what is actually praiseworthy to aspire to. In meaning, the only envy that is acceptable is of two people: one whom Allah has given wealth and who spends it rightly, and one whom Allah has given wisdom and who acts by it and teaches it to others. In other words, admiring and wanting to imitate someone's good use of a blessing, rather than resenting that they have it.

At the centre of tazkiyah is sincerity: doing things for Allah's sake, rather than to be seen or praised. It opens one of the most widely cited hadith collections. In Sahih al-Bukhari, number one: “The reward of deeds depends upon the intentions and every person will get the reward according to what he has intended.” Its direct opposite is showing off, which is warned against sharply. In Sahih al-Bukhari, number sixty-four ninety-nine, the Prophet said that whoever does good things in public to show off and win the praise of people, Allah will disclose his real intention on the Day of Resurrection.

Tazkiyah is not a one-time achievement, but ongoing and unglamorous work. Noticing pride, envy, or a desire to be seen in yourself, and consciously working against it, is itself part of the practice, not a sign of having failed at it. Classical scholarship treats these diseases of the heart, and others such as anger, greed and love of status, in far more depth, and that deeper study is best done with a qualified teacher.