It is commonly held that the sha’n-i-nuzul (occasion of revelation) is in fact the narration of a particular incident or incidents, which spurred the revelation of a verse or set of verses. I am afraid this is not correct. The sha’n-i-nuzul of a revelation in fact refers to the circumstances encompassing the revelation’s addressees at the time when it is revealed.  The Holy Qur’an actually discusses certain matters, or a set of matters, as central themes within the surahs. Each revealed address revolves around that matter or a set of matters. Therefore, the best way to ascertain the sha’n-i-nuzul is to deliberate upon the contents of the surahs themselves, because surahs are always revealed while keeping contextual circumstances in view. Just as a pharmacist identifies a patient’s ailment by simply analyzing the medicines mentioned in the prescription, it is possible to identify the sha’n-i-nuzul of the Qur’anic text by examining the central theme of any particular surah. In a meaningful discourse, the discourse’s content and its theme must possess mutual affinity and be interrelated, just as a well-fitted dress is in accordance with the shape of a body. It goes without saying that the components of a meaningful discourse are closely tied. When a narrative instructs us that a certain verse was revealed with regard to a particular incident, it indicates the circumstances of the addressees at the time of that particular revelation. Thus we come to know of the immediate reasons for the revelation of a surah. Suyuti writes:
 

و قال الزركشي في البرهان: قد عرف من عادة   الصحابة والتابعين أن أحدهم إذا قال: نزلت هذه الآية في كذا, فإنه يريد بذلك أنها تتضمن هذا الحكم, لا أن هذا كان السبب في نزولها, فهو من جنس الاستدلال على الحكم بالآية, لا من جنس النقل لما وقع. قلت : والذي يتحرر في سبب النزول أنه: ما نزلت اللآية أيام وقوعه

Zarkashi writes in Burhan: When the Companions (rta) say that such and such a verse was revealed about such and such an incident, they mean that the verse embodies a ruling about that incident. It doesn’t mean that the verse was primarily prompted by that incident. The narratives are not narrated just in order to report that incident. We in fact deduce a decree from the verse and argue that the verse is giving a directive similar to the one narrated in the respective sha’n-i-nuzul narratives. I believe that it is very important to appreciate that it is not necessary that a verse should have been revealed at the time when the incident took place.¹

    The above quoted saying of Zarkashi solves the problem Razi faced while interpreting ‘…وَ اِذَا جَاءَكَ الَّذيْنَ يُؤمِنُوْنَ بِاياتِنَا’ (6:54) Imam Razi has written:

و لي ها هنا إشكال, و هو: أن الناس اتفقوا أَن هذه السورة نزلت دفعة واحدة, و إذا كان الأمر كذالك, فكيف يمكن أن يقال في كل واحدة من آيات السورة ان سبب نزولها هو الأمر الفلانى بعينه.

I am quite mystified here. The scholars are unanimous that the whole surah was revealed at one time. Then how could one say that such and such verse is prompted by such and such incident?²

    Therefore, in my opinion, as is obvious from the above discussion, all the surahs were revealed to explain matters which needed an explanation, being careful that the surahs’ text is coherent and unambiguous. This is similar to when an expert orator delivers a speech regarding the conditions and requirements around him in such a way that he mentions nothing specific yet his speech covers all the pertinent issues. Likewise, sometimes he mentions a particular person or incident, but his address is all embracing and universal in nature. The same holds true for the revelation of the Qur’an, as is obvious from the Holy Qur’an itself:

وَ إِنْ تَسْئَلُوْا عَنْهَا حِيْنَ يُنَزَّلُ الْقُرْآنُ تُبْدَ لَكُمْ (5: 101)

If you ask about them when the Qur’an is being revealed they will be made plain to you. (5:101)

    This verse testifies to the fact that the Holy Qur’an during the time of its revelation would answer queries that arose in the minds of its addressees without disturbing the flow of its discourse. So when a surah would be completed to meet the objectives of the discourse, it would not be insufficient regarding the clarification of a matter, nor would it contain any excess material.
    Sometimes the need for instruction on a particular matter was not fully met in one surah, which necessitated the revelation of another surah. The ‘occasion of revelation’ would be the same but the style would be different, hence avoiding monotony and dullness. That’s why the surahs revealed in the beginning deal with subjects like resurrection, monotheism, belief in the Messenger of Allah and many other issues, which were similar in nature, but whose style of revelation often differed at different points. Sometimes it was felt that a particular element of a surah needed a further explanation so an explanatory verse would be revealed and placed there. This was in accordance with the promise Allah made in Surah Qiyamah:

ثُمه َإِنَّ عَلَيْنَا بَيَانَهُ (75: 19)

Then upon Us is its explanation. (75:19)

    In such cases, the gathered discourse would follow the context of the surah rather than its period of revelation. Usually such clarifying verses would be followed by a revealed statement making it clear that that part was revealed in order to clarify a matter. Therefore, verses similar to the following would be placed at the  end of the clarifying directive:

كَذَلِكَ يُبَيِّنُ اللَّهُ آيَاتِهِ لِلنَّاسِ لَعَلَّهُمْ يَتَّقُونَ (2:187)

Allah explains his verses to make people understand so that they may be fearful. (2:187)

    If we want to be utterly clear regarding the explanation of a certain part of the Qur’an, we should not let go of the context of the verses lest we become like a desert traveler who gets to a cross-road in the dark of night and doesn’t know where to go. Thus the sha’n-i-nuzul of a surah should be determined from within the surah. Only those narratives should be considered worthwhile in this regard which are in harmony with the context of the surah rather than those which disrupt its coherence. Therefore, the best conclusion regarding the utility of a sha’n-i-nuzul is that which is derived from the Qur’anic context. One should hold tight to it. When a general decree is revealed on a certain occasion this (occasion and condition) alludes to the reason and wisdom behind that decree. For instance the Holy Qur’an mentions both monogamy and polygamy. If we consider the sha’n-i-nuzul by reflecting on the context, it becomes clear that the first decree is related to justice with orphans and the second with wives. The two directives themselves are based on the underlying principle of justice with the weak. Circumstances will tell which one is applicable in a particular situation.