Every human being is born with a conscience. Within this conscience resides the cognizance of good and evil. Through this conscience a person has an inborn knowledge of what is good and what is evil. Not only does a person has the ability to discriminate between good and evil, he is also equipped with an internal correcting mechanism: The conscience within him praises him on every good he does and pricks him on every evil that emanates from him. This is the basic function of human conscience. If a person does not pay heed to the calls of his conscience, it becomes weaker and ultimately even dies. A dead conscience means that it no longer rebukes a person on any wrong he commits. Conversely, it remains alive only when its calls are heeded to.
Now it is the verdict of sense and reason that a person should succeed if he adopts the good revealed to his conscience, and be doomed if he adopts the evil revealed to it. However, since this world is a place of trial and test, and as a consequence of this, the result of a good enterprise is not necessarily good and the result of an evil undertaking is not necessarily evil, a day must come when results are in accordance with the nature of deeds. Furthermore, if the Almighty has no intention of evaluating a person, why at all has He endowed him with such an internal mechanism of correction? The Qur’ān stresses that not believing in a day in which good and evil shall produce congruous results would mean that this world is the toy-land of an unjust Creator in which the righteous and the wrongdoers meet the same fate. It, therefore, asserts that the mere existence of conscience in a person is evidence enough for the Day of Judgement:
[They think that the Day of Judgement will never be]; By no means! I present as evidence the Day of Judgement itself. By no means! I present as evidence this reproaching soul [within you]. Does man think that We will not be able to bring together his bones? Why not? We can put together his very finger tips. [No this is not so]; in fact [the truth is that] man wants to be mischievous before his [conscience]. He asks: ‘When will the Day of Judgement be?’ (75:1-6)
In other words, this chiding conscience within a person shows that he will not be left unaccountable for his deeds. One day, he will be called to account. A person may be blind to the brimming evidence of this Day in the world around him but he cannot be blind to the world within him, unless of course he has lulled the calls of his conscience to sleep.