The “knife-edge theory” says that what caused the decline of science in Islam was the influence of the Asharites and in particular of al-Ghazali. 

Though al-Ghazali faced some criticisms among Muslim scholars—an example of this criticism is the book The Incoherence of the Incoherence, written by the Muslim philosopher Averroes (Ibn Rushd) —the critics did not prevail. Instead, al-Ghazali was—and still is—honored like few other Muslims. He was revered as a “renewer of the faith” and as the “Proof of Islam”, showing that wide segments of the Muslim population were highly receptive for his ideas. This enthusiastic reception by Muslim society suggests that al-Ghazali and the Asharites touched on ideas deeply embedded in Islamic culture and in the religion of Islam.

If this is so, we should find support for the Asharites’ views in Islamic religious texts. And, indeed, we do not have to look long to find such support: Islamic scripture contains places which directly support al-Ashari’s and al-Ghazali’s view of God as the omnipotent creator of the world who, not constrained by any other power, does whatever he wants with his creation, whenever he wants it. He knows and controls everything, as stated in Surah 41:47 of the Quran :

On Him devolves the knowledge of the Hour, and no fruit emerges from its covering and no female conceives or delivers except with His knowledge …

Similarly in Surah 31:34:

Indeed the knowledge of the Hour is with Allah. He sends down the rain, and He knows what is in the wombs.

Allah is the cause of everything: if a Muslim thinks that it was he himself who killed his enemies in a battle, Surah 8:17 teaches him otherwise:

You did not kill them; rather it was Allah who killed them; and you did not throw when you threw, rather it was Allah who threw …

(The ‘throwing’ refers to an incident during one of the battles of Mohammed, in which he threw a handful of sand at the enemy which then apparently blinded the enemy soldiers).

Surah 2:284 is a good example of the arbitrary nature of God:

[Allah] will forgive whomever He wishes and punish whomever He wishes, and Allah has power over all things.

Even Muhammad is said to have told that he wasn’t certain of entering Paradise after his death (according to Sahih Bukhari’s Hadith collection, Volume 5, Book 58, Number 266). God alone decides that. A Muslim can hope for mercy but can not rely on God about anything, with the possible exception of Muslims who die in Jihad, the Holy War against unbelievers—they are guaranteed entering Paradise, according to Sahih Bukhari in Hadith Volume 4, Book 52, Number 44.

A Muslim lives in uncertainty regarding his personal life and life in general. Just as al-Ashari stated, even his choices are determined by God—leaving little room for a “free will”—, as Surah 6:125 says:

Whomever Allah desires to guide, He opens his breast to Islam, and whomever He desires to lead astray, He makes his breast narrow and straitened as if he were climbing to a height.

Surah 14:4 states the same:

Then Allah leads astray whomever He wishes, and He guides whomsoever He wishes, and He is the All-mighty, the All-wise.

Of course, like in other holy texts, there are many contradictions in the Quran, and thus we find verses (e.g. Surah 4:57) which seem to contradict the above, and in which Allah promises Paradise to Muslims who do “good deeds”.

The well known Muslim fatalism (“Inshallah” = “if Allah wants it”) is an expression of this uncertainty and of the recognition of the omnipotent arbitrariness of God. Such a fatalism is actually demanded by the Quran, in Surah 18:23-24:

Do not say about anything, ‘I will indeed do it tomorrow’, without [adding], ‘if Allah wishes.’

However, God’s omnipotence is such that he is not bound by anything, even by his own promises, as Sayyid Qutb, one of the fathers of current Islamic radicalism, notes in his book In the Shade of the Quran:

Every time the Qur’ān states a definite promise or constant law, it follows it with a statement implying that divine will is free of all limitations and restrictions, even those based on a promise or law from God. For His will is absolute. (Qutb, Vol.18, p. 127).

Qutb refers here to Surah 87:6 and Surah 87:7. The first verse states a promise by Allah, given to Mohammed:

We shall have you recite [the Qurʾān], then you will not forget [any of it]

The second verse – directly following the first one – emphasizes that this promise can be broken, if Allah wants it to be broken:

except what Allah may wish. Indeed He knows the overt and what is hidden.

Though there might be verses in the Quran contradicting the above verses, there was ample opportunity for people like al-Ashari and al-Ghazali to build a picture of Allah as an almighty, all-controlling God who has unlimited freedom of meddling in the world and who leaves no freedom for the world to change or develop on its own and no freedom of choice for humans, either. Given that this view of God is based on Islam’s holy scriptures, it is a plausible assumption that these scriptures are ultimately at the root of Islam’s problematic relationship with science and philosophy. The role of al-Ashari and al-Ghazali was merely to explain and emphasize these ideas in a radical but rational manner and to apply them to problems like causality in nature, “free will” and ethics.

Thus, blaming the Asharites and al-Ghazali in particular for the demise of science in Islam is short sighted. Aristotelian natural philosophy was in conflict with all religious forces in Islam: with the Asharites but also with traditionalists like the Hanbalites and even with the Mu’tazilites. The Hanbalites were predeterministic, and the Mu’tazilites and Asharites assumed an atomistic occasionalism (though the Mu’tazilites made an exception for human free will). None of them allowed Aristotelian natural causes.

Thus, it wouldn’t have helped much for the future of science in Islam if, as Reilly (2011) assumes, the Mu’tazilites had won the debate against the traditionalists and the Asharites. For science to flourish in a society, the fundamental view point is necessary that nature has its own causes which can be found out by scientific research. Such a foundation was presented by Aristotelian natural philosophy. However, this philosophy was only practiced by the falasifa. 

The pressure on the falasifa, which included physical threats and other attacks, made sure that natural philosophy disappeared from the intellectual landscape of Islam over time. What remained in its place was religion and the ideas of Ibn Hanbal, al-Ashari and al-Ghazali about a God who predestines all events in the world and exerts a moment-by-moment control about everything in it.