How can there be abrogation in the Quran?

I have heard from many sources, one of them is not Muslim, who say that what is revealed in the later sura's in the Qur'an supersede the earlier surahs. When I heard this I was extremely skeptical, however I thought I would ask you sidi just to confirm this...

Among the verses in the Quran containing orders or laws there are verses that abrogate verses previously revealed and acted upon. These abrogating verse are called _nasikh_ and those whose validity they terminate are called _mansukh_.

The common notion of abrogation, that is, canceling of one law or code by another, is based on the idea that a new law is needed because of a mistake or shortcoming in the previous one. It is clearly inappropriate to ascribe a mistake in law-making to God, Who is perfect, and whose creation admit of no flaws.

However, in the Quran, the abrogating verses mark the end of the validity of the abrogated verses because their heed and effect was of a temporary or limited nature. In time the new law appears and announces the end of the validity of the earlier law. Considering that Quran was revealed over a period of twenty-three years in ever-changing circumstances, it is not difficult to imagine the necessity of such laws.

It is in this light that we should regard the wisdom of abrogation within the Quran:

"And when we put a revelation in place of (another) revelation and Allah knows best what He reveals -- they say: you are just inventing it. Most of them do not know. Say: The Holy Spirit (Gibril) has revealed it from your hand with truth and as a guidance and good news for those who have surrendered (to God)" [16:101-102]

It is a science on its own in Islam to know the Nasikh and Mansukh.

Hajj Gibril