Introspection after Making a Mistake
Introspection is a major source of tazkiya. Introspection awakens the human mind, it jolts the human personality, it gives the individual the incentive to reform himself. In this way introspection leads one to intellectual and spiritual development.
For instance, suppose someone said something which hurt you, you were provoked and you reacted negatively. Later, you became repentant and engaged in introspection, which led you to think that by having behaved in this way, you were developing a negative personality, such as would have a deleterious effect in the life after death. Such a negative personality would, indeed, render one incapable of gaining entry into Paradise.
You came to realize that the culture of the people of Paradise would be that of peace; only those would be accommodated there who had the capacity to live together in love and peace. This being so, one who develops a personality which is intolerant, easily provoked and downright unfriendly will be held ineligible to enter Paradise and will be deprived of happiness and success for all eternity.
Such thinking will prove to be revolutionary and you will become your own watchdog. You will become extremely keen to reform yourself.
Thinking born of introspection is the greatest source of tazkiya. Tazkiya can always be achieved by inner self-appraisal, rather than by some external activity.