When considering the source of the Qur'an, Muhammad is an obvious suspect.
There are however several reasons why this would not make sense:
I will consider these reasons in my answer.
Linguistic and Stylistic Differences Between Muhammad's Speech and Qur'an
The different linguistic and stylistic texts of the Qur'an and the Sunnah prove Prophet Muhammad can't be the author of the Quran.
In 2012, a study was issued by Literary and Linguistic Computing journal sought to identify whether the Qur'an was authored by Muhammed (saw) based on computational based author discrimination.
Scholars use this methodology to identify the authorship of books attributed to a certain author. For example, regarding the New Testament, scholars compare the letters attributed to Paul and concluded all the letters can't be authored by the same author because the linguistic structure and other features are dramatically different.
The study commenced with sixteen experiments compared expressions, citation of animals, similar vocabulary and characters, citation of numbers and animals, special ending bigrams, and others features used in the hadiths (Bukhari) and in the Qur'an. They found dramatical differences between the two books which lead to the conclusion 'the two books must have two different authors.
The study says regarding the types of experiments:
"Thus, three series of experiments are done and commented on. The first series of experiments analyses the two books in a global form ... It concerns nine different experiments. The second series of experiments analyses the two books in a segmental form (four different segments of text are extracted from every book). It concerns five different experiments. The third series of experiments makes an automatic authorship attribution of the two books in a segmental form by employing several classifiers and several types of features. The sizes of the segments are more or less in the same range (four different text segments ...). It concerns two different experiments. … in fact, all the results of this investigation have shown that the two books should have two different authors.
.... the Quran was not written by the Prophet Muhammad and that it belongs to a unique author too." (H. Sayoud, Author discrimination between the Holy Quran and Prophet's statements, in Literary and Linguistic Computing 27(4):427-444 ·(2012) DOI: 10.1093/llc/fqs014)
Muhammad's Elevated Moral Conduct
Muhammad was known by his tribes as "the most honest and trustworthy person" (al-Sadiq al-'Ameen) amongst them.
When he claimed revelation from Allah at the age of 40, some believed his claims due to such attributes.
Abu Bakr is one such example from amongst his close friends. Some even believed in him without even seeing him, people of Medina spring to mind.
The criterion of trustworthiness is practically widespread in our daily life; we trust moral people, teachers, scholars, experts and others especially when we have seen how honest and faithful they were in many incidents.
This point emerged when Allah ordered him to preach the Massage publicly. Muhammad ascended mountain Safa and addressed his people:
أَرَأَيْتُمْ إِنْ أَخْبَرْتُكُمْ أَنَّ خَيْلاً تَخْرُجُ مِنْ سَفْحِ هَذَا الْجَبَلِ أَكُنْتُمْ مُصَدِّقِيَّ ". قَالُوا مَا جَرَّبْنَا عَلَيْكَ كَذِبًا. قَالَ " فَإِنِّي نَذِيرٌ لَكُمْ بَيْنَ يَدَىْ عَذَابٍ شَدِيدٍ
"What if I inform you that cavalrymen are proceeding up the side of this mountain, will you believe me?" They said, "(yes) We have never heard you telling a lie." Then he said, "I am a plain warner to you of coming severe punishment." (Bukhari, Hadith 4971)
It remains unpersuasive that a man who had never lied to his own people would lie against the All-Knowing, All-Hearing and Omnipotent Creator.
Muhammad unable to Understand some Revelation
When one says the Prophet authored the Qur'an, it implies he knows or understands the meanings behind verses he states. On the contrary, the Prophet did not know the purpose of certain revelations: when Allah revealed:
وإن تبدوا ما في أنفسكم أو تخفوه يحاسبكم به الله
"Whether you show what is within yourselves or conceal it, Allah will bring you to account for it." (Qur'an 2:284)
The companions asked the Prophet 'How can we be judged for something we did not do?' The prophet did not know its intended meaning and told them 'Say, we hear and we obey.' (Sahih Muslim) Later a verse was revealed to clarify its meaning:
آمَنَ الرَّسُولُ بِمَا أُنزِلَ إِلَيْهِ مِن رَّبِّهِ وَالْمُؤْمِنُون ... لَا يُكَلِّفُ اللَّهُ نَفْسًا إِلَّا وُسْعَهَا ۚ لَهَا مَا كَسَبَتْ وَعَلَيْهَا مَا اكْتَسَبَتْ
"The Messenger has believed in what was revealed to him from his Lord, and (so have) the believers. … Allah does not charge a soul except [with that within] its capacity. It will have (the consequence of) what (good) it has gained, and it will bear (the consequence of) what (evil) it has earned." (Qur'an 2:285-286)
This could not be justified by saying 'he may have forgotten the previous revelation,' because some revelations were just revealed and he did not understand their intended meaning. The previous revelations were remembered, but the point here is focussing on the understanding or a given verse.
Muhammad Misunderstanding some Revelation
If the Prophet was the real author of the Qur'an, why did he misunderstand what he just said? The Qur'an orders him regarding the death of hypocrites:
اسْتَغْفِرْ لَهُمْ أَوْ لاَ تَسْتَغْفِرْ لَهُمْ إِن تَسْتَغْفِرْ لَهُمْ سَبْعِينَ مَرَّةً فَلَن يَغْفِرَ اللّهُ لَهُمْ ذَلِكَ بِأَنَّهُمْ كَفَرُواْ بِاللّهِ وَرَسُولِهِ وَاللّهُ لاَ يَهْدِي الْقَوْمَ الْفَاسِقِينَ
"Ask forgiveness for them, (O Muhammad), or do not ask forgiveness for them. If you should ask forgiveness for them seventy times - never will Allah forgive them. That is because they disbelieved in Allah and His Messenger, and Allah does not guide the defiantly disobedient people." (Qur'an 9:80)
When Ibn Saba' (the leader of the hypocrites) died, the Prophet (saw) wanted to lead the funeral prayer. When Umar (ra) objected and reminded him of the verse, he said:
أَخِّرْ عَنِّي يَا عُمَرُ ... إِنِّي خُيِّرْتُ فَاخْتَرْتُ، لَوْ أَعْلَمُ أَنِّي إِنْ زِدْتُ عَلَى السَّبْعِينَ يُغْفَرْ لَهُ لَزِدْتُ عَلَيْهَا
"Leave me alone, O 'Umar. … I have been given the choice and I have chosen (to offer the prayer for him). If I knew that he could be forgiven by asking Allah's forgiveness more than seventy times, I would have done so." (Sunan an-Nasa'i 1966)
The Messenger of Allah (saw) then offered the funeral prayer for him, then a verse was revealed clarifying the meaning more for him:
وَلاَ تُصَلِّ عَلَى أَحَدٍ مِّنْهُم مَّاتَ أَبَداً وَلاَ تَقُمْ عَلَىَ قَبْرِهِ إِنَّهُمْ كَفَرُواْ بِاللّهِ وَرَسُولِهِ وَمَاتُواْ وَهُمْ فَاسِقُونَ
"And do not pray (the funeral prayer, Muhammad), over any of them who has died - ever - or stand at his grave. Indeed, they disbelieved in Allah and His Messenger and died while they were defiantly disobedient." (Qur'an 9:84)
Intermittency of the Revelation
The revelation was intermittent or withheld in a number of urgent situations, something if he was the source he could have easily remedied by issuing some verses.
At the beginning of his mission, the revelation was stopped (Bukhari 305) until the Prophet began thinking Allah had left him. Even some of his enemies said your devil has forsaken you. After a while, some narrations say months, a small chapter was revealed; Surat ad-Duha 93.
The same thing happened for other cases such as:
If he authored the Qur'an, it seems unusual to allow people to criticize him for something he could easily avoid. Why did not he issue the revelation from the very beginning? The most plausible response would be he was not the author.
On a number of occasions, the Qur'an reprimanded, censured or blamed the Prophet for certain acts. Such critique appears to emerge from a third party against him - not critique he would impose upon himself
Psychological and Emotional Variances
The Prophet (saw) experienced many collective hardships and trials in a very short time which made his life grief-ridden.
In one year alone his beloved wife Khadijah and his uncle Abu Talib, who used to protect him, passed away after expulsion and boycott from Mecca.
He and his companions (ra) had been tortured severely.
All of his daughters and sons passed away in life except one daughter.
None of these turmoils and grievances were expressed in the Qur'an. If Muhammad was a poet, it seems strange why he did not express his feelings in the Qur'an.
In addition, the names of his beloved wives, closest people such as his parents, his children, earlier friends and converters..etc) are not mentioned more than four times in the Holy Qur'an.
The content of the Qur'an is full of religious, cultural and historical references of the previous and contemporary nations. Such information requires considerable learning requiring significant self-study of detailed historical literature or acquisition from a well-informed scholar. Studies of the places where the Qur'an was revealed and where the Prophet (saw) lived or visited did not contain any such sources. So we can only conclude it is implausible to attribute the content of the Qur'an to any available human source; it can only have come from a source that is beyond human capability.
Whilst the Qur'an prophesied various matters, more significant and relevant matters were never prophesied.
For instance, the Qur'an prophesied the defeat of Persia:
غُلِبَتِ الرُّومُ، فِي أَدْنَى الْأَرْضِ وَهُم مِّن بَعْدِ غَلَبِهِمْ سَيَغْلِبُونَ، فِي بِضْعِ سِنِينَ ۗ لِلَّهِ الْأَمْرُ مِن قَبْلُ وَمِن بَعْدُ ۚ وَيَوْمَئِذٍ يَفْرَحُ الْمُؤْمِنُونَ
"The Byzantines have been defeated in the nearest land. But they, after their defeat, will overcome within three to nine years. To Allah belongs the command before and after. And that day the believers will rejoice" (Qur'an 30:2-4)
But when it came to the date of the day of judgement, the Qur'an says:
يسْأَلُونَكَ عَنِ السَّاعَةِ أَيَّانَ مُرْسَاهَا قُلْ إِنَّمَا عِلْمُهَا عِندَ رَبِّي لاَ يُجَلِّيهَا لِوَقْتِهَا إِلاَّ هُوَ ثَقُلَتْ فِي السَّمَاوَاتِ وَالأَرْضِ لاَ تَأْتِيكُمْ إِلاَّ بَغْتَةً يَسْأَلُونَكَ كَأَنَّكَ حَفِيٌّ عَنْهَا قُلْ إِنَّمَا عِلْمُهَا عِندَ اللّهِ وَلَـكِنَّ أَكْثَرَ النَّاسِ لاَ يَعْلَمُونَ
"They ask you, (O Muhammad), about the Hour: when is its arrival? Say, "Its knowledge is only with my Lord. None will reveal its time except Him. It lays heavily upon the heavens and the earth. It will not come upon you except unexpectedly." They ask you as if you are familiar with it. Say, "Its knowledge is only with Allah, but most of the people do not know."" (Qur'an 7:187)
If the Qur'an is authored by Prophet Muhammad (saw) who is certainly not God, why would he take the risk of predicting something that could undermine his claims of prophethood within a decade (the defeat of Persia), whilst not answering a question that could not be disputed for potentially thousands of years after his death?
The simplest answer to all these questions is:
قلْ مَا يَكُونُ لِي أَنْ أُبَدِّلَهُ مِن تِلْقَاء نَفْسِي إِنْ أَتَّبِعُ إِلاَّ مَا يُوحَى إِلَيَّ إِنِّي أَخَافُ إِنْ عَصَيْتُ رَبِّي عَذَابَ يَوْمٍ عَظِيمٍ
"Say, (O Muhammad), 'It is not for me to change it on my own accord. I only follow what is revealed to me. Indeed, I fear, if I should disobey my Lord, the punishment of a tremendous Day.'" (Quran 10:15)
There are various reasons why the author of the Qur'an is unlikely to be the Muhammad (saw) ranging from linguistic differences, Muhammad's moral conduct through to the Qur'anic censure of his acts and an absence of his psychological and emotional states in the Qur'an.
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