Hey! Interesting question. I don't know exactly what book you're reading, but I'd wager what it is referring to is known as the Closing of the Gate of Ijtihad.

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#What is Ijtihad?

Ijtihad is the process used by Muslim scholars to figure out the answer to a question that is not explicitly stated in the Quran or hadiths, pulling on similar rules or cases, analogy, and their own reasoning. This is composed of multiple parts - qiyas (analogy), 'ijma' (Consensus), precedent, and more - which I won't dive too far into. Needless to say, it was a rather methodological process, not merely a village imam declaring something to be haram or halal. In fact, in order to practice ijtihad you had to be a mujtahid. This entails not only knowing all of the Quran and Hadith, but also being versed in the particularities of Arabic, the common law applied in your area, the principles of inference (istidlal), and being considered a reputable scholar in your own right.

In some ways, this can be seen as analogous to how a modern judge in Common Law countries will interpret the law and apply it to a novel case. For example, the mujtahid may face a question. Is smoking permissible? He will then look at the Quran and hadith, where it isn't explicitly banned. However, he may remember that the it intentionally harming your own body is prohibited, and smoking harms your own body. By applying the principle to the case, he may then rule that smoking isn't permissible in Islam.

#Ok, so why did Ijtihad stop happening?

Some groups had been against ijtihad from the very death of Muhammad, believing that relying on human reason in any way means relying on the fallible. Instead, one should stick to taqlid, or imitation. However, these were mostly marginal, fringe groups and ijtihad continued unabated in most Sunni schools of thought.

There soon emerged a conflict between more traditionalist jurists and more rationalist jurists. While the Traditionalists preferred to rely upon hadith to come to answers that aren't explicit in the Quran, the Rationalists preferred to rely on ra'y, (independent reasoning). Another way of thinking about this is that the Rationalists tried to "articulate the message of Islam within any given age's contemporary intellectual and social trends" (Richard Martin)

Perceived over-reliance on personal reasoning led many Traditionalists to fear, like those fringe sects before, that Islam was at risk of being perverted. As such, there was a movement away from ijtihad and in favor of taqlid. Majid Khaddury wrote about this that:

"As a result, ijtihad was gradually abandoned in favour of taqlid .... At first taqlid reflected a tendency to reduce the differences among jurists and limit the number of schools, but the growing intolerance of the Muslims to legal reasoning rendered taqlid an obstacle to legal development. Thus the door of ijtihad was shut and taqlid became the dominating rule"

The final step comes through the aforementioned 'ijma', or consensus. There is a hadith that claims ""Indeed Allah will not gather my Ummah . . . upon deviation, and Allah's Hand is over the Jama'ah, and whoever deviates, he deviates to the Fire."" This was widely taken to mean that, if the Islamic community came to a consensus, it could never be wrong. As taqlid gradually overtook ijtihad, by the 9^th / 10^th century, many claimed that a consensus had formed that "the Gate of Ijtihad is closed" - there have been sufficient ruling in the past that any questions which need an answer are answerable, and it is no longer necessary to use independent reasoning to come to new solutions.

#So is that why the Muslim world stopped advancing?

Well, there is a lot of disagreement over whether or not the Gate of Ijtihad did close. The prominent scholar Wael Hallaq adamantly denies that ijtihad ever stopped occurring, only that its usage fluctuated throughout the centuries. This could be seen in religious trends, theoretical trends (such the large movement that occured during the nahda, or Arabic Renaissance, of the 18^th and 19^th centuries to reconcile Islam with modern developments), and even political trends, with Habib Bourguiba, the leader of Tunisia from 1956 to 1987 invoking ijtihad to justify his modernization and secularization of the country.

Also, it is important to note that not all Sunni schools of thought accepted the end of ijtihad, let alone the Shi'a, who have continually employed ijtihad since the 7th century CE.

So when your book argues that this was the reason for the stagnation of the Islamic world, it would be wise to take it with a grain of salt. The actual action of closing the Gate of Ijtihad is only part of a more general trend towards imitation of the past. The reason for why the Muslim world ended up so behind Europe, when for centuries it had been ahead, is a much larger and more complicated question that scholars have been trying to answer for hundreds of years. Going into detail about the various theories, from both Muslims and non-Muslims, has filled countless books so I'm not even going to try and tack it on to the end this question.

So in conclusion, your book is probably referencing the "Closing of the Gate of Ijtihad", alleged to have taken place around the 9^th to 10^th century CE, which was part of a more general Islamic trend towards imitation of the past. Although commonly cited, it has come under considerable scrutiny recently and is only one part of the changes in development in the Muslim world.

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I hope that this helped to answer your question. If I was wrong about assuming your book referred to the Gates of Ijtihad, sorry for writing such a long post but I hope you learned something anyways! If you have any follow-ups, please feel free to ask and I'll try and get back to them when I can.

Edit: Forgot to put in my sources!

Main Sources:

Hallaq, Wael B. “Was the Gate of Ijtihad Closed?” International Journal of Middle East Studies, vol. 16, no. 1, 1984, pp. 3–41. JSTOR, JSTOR, www.jstor.org/stable/162939.

(For more about it, see Hallaq's book Sharia: Theory, Practice and Transformations. Edinburgh, UK: Cambridge University Press, 2009. )

Ali-Karamali, Shaista P., and Fiona Dunne. “The Ijtihad Controversy.” Arab Law Quarterly, vol. 9, no. 3, 1994, pp. 238–257. JSTOR, JSTOR, www.jstor.org/stable/3381568.

Martin, Richard C., Mark R. Woodward, and Dwi S. Atmaja. Defenders of Reason in Islam: Muʻtazilism from Medieval School to Modern Symbol. Oxford, England: Oneworld Publications, 1997.