The relationship between certain sciences and the Holy Quran is in such a way that without those sciences it is not possible to interpret and analyze the verses of the Holy Quran. For example the sciences of ‘Sarf and Nahw’, ‘Ma’aani and Bayaan’, and ‘Luqat’ are all practical examples. Hence, a commentator should have mastery in all of the sciences which relate to a better understanding of the Holy Quran. In the Quran there are cases which are general and specific, absolute and particular, abrogated and abrogating; for understanding these subjects deeply, it is necessary to have a profound understanding of the Quranic verses. When the verses are understood completely then it becomes clear which verses are absolute, which are general, and which are abrogated. For this reason the Imams (a) are the best commentators and interpreters of the Holy Quran, because they are in reality the ‘Living Quran’.
From one perspective, the ban on interpretation based on opinion does not mean a rigidity of thought; in fact, a lack of thinking has been censured in the Holy Quran. God says in the Quran: “Do they not ponder over the Quran? If it was from other than God, you would find many contradictions within it.”[i] God, in this verse, has introduced the reason behind the deviation and ignorance of the polytheists as being the lack of thinking and reflection. Interpretation based on opinion which is frowned upon in Islam, is an interpretation where an individual, without paying any attention to traditions and the interpretation of the Infallibles (a) and without paying attention to the other verses of the Quran (general and specific, absolute and particular, abrogated and abrogating), and without having sufficient mastery over the sciences which relate to commentary, goes ahead according to his own opinions and ideas, and interprets and analyzes the verses of the Holy Quran. In essence he gives his own ideology and desires to the meaning of the Quran.
[i] Surah Nisaa, Verse 82
Before delving into the main topic itself, there are several points which will help us in reaching the main points of the answer.
1. The majority of the verses of the Quran were revealed for the understanding of the general masses. From this perspective, the understanding that the masses have from the literal and apparent meaning of the Quranic verses are a proof and it is for this same reason that the ‘Usulis’ believe in the external meaning of the verses.
2. Of course there are also select verses in the Quran whose understanding and the helping of others to understand was the responsibility of the Prophet (s) and the ‘Rasikhoon Fil Ilm’ (those deeply endowed with knowledge). Without this group’s assistance and help, the understanding of these verses would have been impossible to attain.
3. In the Quran there are verses with meanings of general and specific, absolute and particular, abrogated and abrogating; in order to understand the Quran it is necessary to have a comprehensive knowledge of all Quranic verses to prevent mistakenly ruling according to the absolute without considering the other verses that might narrow its scope.
4. There are some sciences, which although they don’t have a direct connection with interpretation of the Holy Quran, have a prerequisitational relationship. Without understanding these sciences it is not possible to interpret and analyze the verses of the Quran. Examples of these sciences are ‘Sarf and Nahw’, Ma’aani and Bayan’, and ‘Luqat’. The commentator must have mastery over all of the sciences which relate to the understanding of the Holy Quran. Now that these points have been introduced, we will move onto answering you question.
Interpretation based on self opinion and rigidity of thought
The prohibition on interpretations based on opinion does not mean that one should be intellectually rigid. In fact it is quite the opposite. The Quran has shown the reason of the deviation of a certain group to be their lack of thinking and reflection and has condemned the people who do not involve themselves in these essential activities. It says: "أَ فَلا یَتَدَبَّرُونَ الْقُرْآنَ أَمْ عَلى قُلُوبٍ أَقْفالُها"
Likewise God has praised those who derive new matters from the Quran and he has said: "لَعَلِمَه الَّذِینَ یَسْتَنْبِطُونَهُ مِنْهُمْ"”. In another verse he has said: “Do they not ponder over the Quran? If it was from other than God, you would find many contradictions within it.
God, in this noble verse in the Quran, has introduced the reason behind the deviation and ignorance of the polytheists as their lack of thought and reflection. The Holy Prophet (s), in the event of Ghadir Khum after his last Hajj, told the people: “Oh People, pay attention and reflect on the Quran. Understand its verses, pay attention to its decisive verses (muhkamaat) and do not go after its allegorical verses (mutashabeh) [do not focus only on its allegorical verses without considering the decisive ones to correctly interpret them]. From this perspective the Quran and traditions encourage people towards reflection and thinking and without doubt the interpretation and analysis of the Quran is also included in this.
Interpretation based on opinion, which has been censured by the leaders of our religion, is an interpretation that an individual does without paying attention to traditions, other interpretations, and the general principles which relate to the verses of the Quran. In essence it is an interpretation derived in a vacuum without paying attention to its context. This also includes not paying attention to the other verses of the Quran (general and specific, absolute and particular, abrogated and abrogating). In essence this is an interpretation according to an individual’s own ideas and opinions which he holds in interpreting and analyzing the verses of the Quran; it is in fact the act of someone translating their own ideology onto the Quran.
Such an interpretation is not acceptable in Islam and it has been explained that whichever individual speaks about the Quran without knowledge, his place will be the hellfire.
So through paying attention to the points that we just explained, if someone attempts to interpret the Holy Quran according to their own desires and whims without having the necessary prerequisite knowledge of the sciences of interpretation (such as Usul, Luqal, Nahv, and Hadith), then he has created a rift between himself and God and according to the words of the Prophet (s) he has made himself a place in the Hell. On the other hand if he interprets based intellectual thinking that is based on the Usul and religious standards and principles, then this is not considered interpretation based on opinion.
One of these Usul (principles) is paying attention to the station of the Infallibles (a). This is required because the Quran has made the having of purity (taharat) before ‘touching’ the book an essential condition. This means both external and internal purity. Just like the physical touching of the Quran requires purity (taharat), the ‘internal touching’ of the essence of the Quran also requires taharat of the internal nature of the human being. The more pure someone is, the more deep they can reach the realities of the Holy Book. The Imams (a) are pure and infallible from sins and have the conditions of complete purity (taharat) which extend to both the external and internal aspects of life; they are in reality the ‘Living Qurans’.
In conclusion, people other than the Infallibles (a) can be the interpreters of the Holy Quran if they meet these three conditions: 1. Firstly, they must be proficient and have mastered the sciences which relate to the science of interpretation. 2. Secondly, if there is already commentary from the Infallibles (a) on a specific ayah (which is authentic) they should not go against that.3. Thirdly, their interpretation shouldn’t be based on assumed theories.
 In this regard, you can refer to Question 5144 (website 5370).
 “Do they not contemplate the Qur'an, or are there locks on the hearts?” Muhammad:24.
 “…but had they referred it to the Apostle or to those vested with authority among them, those of them who investigate would have ascertained it” Nisa:83; Tafsir Ithna Ashari, vol. 1, pg. 10.
 Surah Nisaa, Verse 82.
 Tabarsi, Ahmad ibn Ali. Al-Ihtijaj, vol. 1, pg. 60; Tarjomeh az Nasa’eh, pg. 400.
 Saduq, Al-Tawhid, pg. 91.
 “May the station of that whom interprets the Quran according to his own personal opinion be filled with fire”, Razi, Fakhruddin Abu-Abdillah Muhammad ibn Umar, Mafatihul-Gheyb, vol. 7, pg. 148.
 “لا یَمَسُّهُ إِلاَّ الْمُطَهَّرُون” Waqi’ah:79.
 Surah Ahzaab, Verse 33 “ِنَّما یُریدُ اللَّهُ لِیُذْهِبَ عَنْکُمُ الرِّجْسَ أَهْلَ الْبَیْتِ وَ یُطَهِّرَکُمْ تَطْهیراً”