The real source of self-purification is introspection, that is, thinking about oneself, analyzing one’s words and deeds, or in other words, becoming one’s own judge, by thinking against oneself and assessing oneself objectively. And no tazkiya is possible without such introspection.
The most powerful feeling in an individual is that of egoism. This feeling is so intense that every person lives with the notion than he is all in all. This is a form of self-glorification. This kind of attitude should be anathema to seekers of tazkiya. What a person should do is develop in himself anti-self thinking, so that he may stand up to criticism. This feeling was so intense in Umar ibn al-Khattab that he said, “May God bless the man who sends me the gift of my shortcomings.” (Sunan al-Darimi, Athar No. 675)
This introspective mentality is born out of a discovery— that of one’s own helplessness. The seeker of tazkiya must discover the reality that his sense of ‘I’ exists only at the level of his own senses, or feelings. It does not exist in reality; he has no power over anything outside of his own feelings. He exercises no power in the matters of his life and death, the life support system, the divine court, and so on. When a person discovers his total powerlessness, the feeling of helplessness is necessarily born within him. It is this feeling of helplessness which compels one to engage in introspection and it is in this discovery that the real secret of tazkiya lies.
Source: Discovering God