As their links with the new metropolis strengthened, the luminaries of Sura and Pumbedita found they had much in common with their opposite numbers in the Islamic world. They discovered they were grappling with similar issues, actively applying their scholarship, legal and religious traditions, to regulate the day-to-day lives of their co-religionists.

Each faith influenced the other. This is obvious both from the structure of their legal systems and some of the legislation itself. Their influence upon each other was more than just a simple two-way process; Gideon Libson (in Halakhah and Reality in the Geonic Period) explains it as a feedback model in which the talmudic system first impacted Islam, which at a later stage left its imprint on talmudic law.

Both Islam and Judaism are religions which minutely regulate every aspect of the believer’s life. They’re each based on a God-given written document – the Torah for Judaism and the Qu’ran for Islam. These divine texts are each interpreted and expanded upon by an oral tradition – the Talmud and the Hadith respectively. Both traditions contain legal and ethical material, and the legal material in each distinguishes between religious laws and social laws.

The Jewish system of law is called halachah, the Islamic system is called shar’ia. Both names mean a “pathway” or a “way to go”. Unlike Christianity, the laws and beliefs in Islam and Judaism are derived through a process of reasoning and scholarship; there are no councils or synods to rule on doctrine, ethics or behaviour. In fact the two religions are so close in terms of their structure that the tenth-century rabbinic leader Saadia Gaon would unselfconsciously refer to Jewish law as shar’ia, to the prayer leader in a synagogue as an imam and the direction in which Jews faced when praying as qibla.

Although in these twin systems the Qu’ran and Torah parallel each other as divinely revealed written texts, the Qu’ran was written down long after the Torah. So whilst we find characters, ideas and motifs from the Torah in the Qu’ran (eg Sura V.44). we shouldn’t expect to find them the other way round.

However, the Talmud and Qu’ran do originate from a similar period and it’s not hard to find concepts from the Talmud occurring in both the Qu’ran and Hadith, and vice versa.

One such case is the ritual definition of daybreak. The Qu’ran defines the moment of daybreak, when the faithful must begin to fast during Ramadan, as when the “white thread of dawn becomes distinct to you from the black thread”( Sura II.187). Similarly, the Jewish Mishnah rules that the morning prayer is to be said when the worshipper can distinguish between blue and white. Both traditions use the same analogy to emphasize the sanctity and uniqueness of every human life. The Qu’ran states that “We decreed upon the Children of Israel that whoever kills a soul … it is as if he had slain mankind entirely. And whoever saves one – it is as if he had saved mankind entirely” (Qu’ran Sura V.32).

This is a reference to the passage from the Mishnah that: “Adam was created alone to teach you that whoever takes a human life is considered by the Bible to have destroyed an entire world. And whoever saves a human life is considered by the Bible as if he preserved an entire world (Sanhedrin 4.5).

Initially Jewish ideas found their way into Islam but the process’s subsequent reversal can be spotted in matters of finance and commerce. Talmudic law matured in Baghdad in a commercial, Islamic environment and Shmuel had already declared that when it came to civil law “The law of the land is the law”. This gave the geonim flexibility in commercial matters to amend or even abrogate Talmudic sanction as necessary.

We can see an example of this flexibility in the laws governing money transfers. The Talmud had instituted that, as a precaution against fraud, merchants could not transfer money by bills of exchange even when these were countersigned by witnesses.

However a geonic ruling overturned this ruling on the basis that people were already doing it, and that it was in accordance with the Islamic laws that regulated merchants: “It is true that the sages said we should not send money by bills of exchange, even if witnesses have signed them. However, since we have seen that people use them we have begun to accept them in court in order not to impede trade between people, and we give judgement according to the traders’ law; neither more nor less” (Teshuvat Hageonim).

Mark Cohen (in Under Crescent & Cross – The Jews in the Middle Ages) points out that this “shared judgement of Muslim and Jewish legal experts … could only occur in a market atmosphere that knew no confessional boundaries”.

An extensive survey by Gideon Libson has shown similarities between the rulings of a tenth-century gaon, Shmuel ben Hofni, and Islamic legal writing of the same period. Of course, as Libson concedes, the fact that there are similarities between two legal rulings in different systems doesn’t necessarily mean that one system was dependent upon the other; they may both have independently derived their rulings from a third source that they each knew.

But with all the other evidence of contact and cross-influences between the faiths, it’s pretty likely that the Talmudic and Islamic systems of law influenced each other.

Talmudic and Islamic scholars cross-fertilized in legal matters because they lived in the same mercantile society. But the two traditions didn’t just overlap when it came to the law. Story telling was an art in the folklore-rich Arabian world.

Amongst the few things that the patchwork of Jewish sects in the Arabia Peninsula had in common was a repository of folklore. The Talmud had drawn on some of it, but there was much more which it did not absorb, including literature linked to the secessionist priestly sect at Qumran, who are best known as the authors of the Dead Sea Scrolls. Early Islam had drawn in Jewish converts who recounted these Dead Sea tales along with everyone else, based on their memories of legends they had heard as Jews.

As these stories began to circulate they took on an Islamic guise; the more they were repeated in the Islamic world, the more they were adapted to fit the cultural context. This folklore, which became known as Isra’iliyyat, was not always looked upon kindly by Islamic leaders. It frequently came in for fierce censure.

The Egyptian scholar Ahmad Shakir explains that Isra’illiyat literature can only be regarded as supporting mainstream Islamic traditions, it should not be relied on unless it is confirmed by the Qu’ran and Hadith.

Amongst the many Jewish converts whose stories entered Islamic hagiography, two in particular stood out. K’ab al-Ahbar, a Yemenite Jew, is thought to have been one of Caliph Umar’s closest advisers. Amongst the sayings attributed to him is that all human history is alluded to in the Jewish Torah; a Talmudic idea first expressed by the intriguingly named Ben Bag-Bag (Mishnah Avot 5:22).

Another convert, or possibly the son of one, Wahb ibn Munabbih, wrote, or contributed to, a work known as Kisas al-Anbiya, the Tales of the Prophets which recounts Jewish biblical legends, recast in an Islamic guise. Kisas al-Anbiya is considered to be the source for the Islamic belief that Abraham is commanded to sacrifice Ishmael, rather than Isaac as the Hebrew Bible has it. The Qu’ran does not say which son was nearly sacrificed.

Wahb’s other major work, Kitab al-Isra’illiyat, or Book of Jewish Matters, no longer exists but some of its tales appear, in an Islamic context, in the Thousand and One Nights.

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Harry Freedman speaks about his new book at Jewish Book Week in London, 11am March 2