If we take the “It’s what Muslims do, and how they interpret their texts” axiom seriously, then there would be no grounds to declare that a Muslim who believes in a pantheon of gods is unfaithful to the teachings of Islam. After all, the Quran, speaking with the Divine Voice, often uses the royal "We" when addressing Muslims. Would this belief in multiple gods also be ‘Islam’? Would these polytheistic Muslims have “just as much legitimacy as anyone else” because they are drawing on the same texts as other Muslims?

Can we extend the axiom of “There is no X, there is only what followers of X do and how they interpret their texts” beyond Islam? If a scientist claims, “Eugenics is not a valid application of the principles of science, and is unscientific,” should he expect to be told that the eugenicists were “just as legitimate as anyone else” because they are following the same body of texts? Were not the eugenicists “serious” and “assiduous” in their science, at least in their own eyes? Did they not speak the language of science, and base themselves on Darwin?

In fact, no one acknowledges that all interpretations of their own system of ultimate meaning are equally authentic or faithful, whether this system is scientism, communism, post-modernism, or any other metaphysical commitment including religion. It is arbitrary to present the Islamic interpretative tradition as an unrestricted free-for-all where nothing is assessed on objective rational or moral criteria, in which every last impulse or assertion is equal to all other responses and can never be subjected to judgment or ranking.

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What other Muslims have been arguing from the start is that ISIS does not take the texts seriously.

The Quran is a single volume, roughly the length of the New Testament. It is a complex and nuanced text that deals with legal, moral, and metaphysical questions in a subtle and multifaceted way. Then there are the hadīth, or records of sayings and doings of the Prophet Muhammad, which run into dozens of volumes spanning literally hundreds of thousands of texts, each on average a few sentences long. Then there is the juridical and theological literature about the Quran and the hadīth, which consists of thousands of works written throughout Islamic history.

Does ISIS cite “texts”? Yes, though its main method is to cite individual ḥadīth that support its positions. But remember: The ḥadīth consist of hundreds of thousands of discrete items that range from faithfully transmitted teachings to outright fabrications attributed to the Prophet, and every gradation in between.

Over the centuries, jurists and theologians of every stripe, Sunni and Shiite, have devised rational, systematic methods for sifting through ḥadīth, which are often difficult to understand or seem to say contrary things about the same questions. They have ranked and classified these texts according to how reliable they are, and have used them accordingly in law and theology. But ISIS does not do this. Its members search for text snippets that support their argument, claim that these fragments are reliable even if they are not, and disregard all contrary evidence—not to mention Islam’s vast and varied intellectual and legal tradition. Their so-called “prophetic methodology” is nothing more than cherry-picking what they like and ignoring what they do not.