The Quranic term 'tazkiya' basically means re-engineering of the mind. According to the Quran, Paradise is for those who undertake their tazkiya. The process of tazkiya is not a matter of the heart, but of the mind. When a person is born, the process of conditioning begins. That is, he forms ideas, develops habits and gradually becomes conditioned along these lines. Although one is born on nature, the influence of one's environment makes one a conditioned person. Returning to nature is the process of de-conditioning. This is re-engineering or tazkiya. Tazkiya literally means to purify. Thus tazkiya means to reform one's thinking and habits.
