Can a man and woman share a horse?

Some years ago, an Egyptian preacher of my acquaintance so infuriated his congregation that they chased him from his mosque until they reached his home. On breaking down the door, they found to their perplexity that God had enabled him to dematerialise, pass through several walls and join his friends at a nearby coffee-house.

The preacher's offence had been to decry the use of hashish, a substance that makes life more bearable for Cairene taxi-drivers. He may have been foolhardy in risking his congregants' wrath, but he stood in a venerable tradition. From the beginnings of Islam, Muslims have been expected to "change any evil they see with their hands; if they cannot, then with their tongues, and if they cannot, then in their hearts". Such was the advice of the Prophet himself.

It is this duty, koranically known as "commanding right and forbidding wrong", that forms the subject of Michael Cook's book. The lively medieval and modern arguments are chased through 700 pages, not a few of them dominated by footnotes. Cook stacks up his examples to produce the sort of dens-ely referenced scholarship familiar to readers of Peter Brown. Like Brown, he too deploys humour and a sense of empathy that succeed, more often than not, in carrying us easily through his disquisitions on intricate Muslim debates.

Cook's Muslims were worried about a large number of public wrongs. They broke wine-jars and acted to prevent sexual display, whether flagrant or implied, as in the case of a man and woman sharing a horse. They held long discussions about the extent to which one might intervene to prevent immodesty in the public baths, to interrupt excessively long sermons, or to prevent the maltreatment of animals. Unlike many of their contemporaries in other religions, Muslim saints tended to be celebrated for their hands-on attitude to society, as they saved women from rape, protected Christians from abuse, and arrested highwaymen.

Some were gentle and "commanded the good" simply through their pious demeanour; but others were feared for their militancy, such as the formidable woman who patrolled the marketplace of Medina with a whip, on the lookout for any violations of God's law. Zealots from the extreme Wahhabi sect that conquered Jedda in 1925 went so far as to confiscate mouth organs from boys in the street. Members of the same sect thought it proper to flog an Egyptian chauffeur for smoking a cigarette.

As Cook shows, such social activism was forced to negotiate with several other Islamic virtues. A believer's home was usually his castle, and "curtain-ripping" or climbing over walls to discover furtive peccadillos was frowned upon. One early pietist was asked what could be done about a man who appeared in public with wine dripping from his beard. He responded curtly that "God has forbidden spying". A woman saint thought that "whoever reproaches his brother secretly will improve him; whoever does it in public will only shame him". There were also arguments about the Prophet's teaching that "the best jihad is to speak the truth in the presence of a tyrant". Some shied away from the wrath of sultans, noting that "the swords of the rulers are mightier than our tongues". Others perfumed and shrouded themselves before preaching to rulers, confident that their reproaches would be rewarded with martyrdom. In some cases, "commanding the right" could lead to outright rebellion.

Cook is alert to the issues this raises for comparative law, and notes that Islamic law is unique in allowing that "every legally competent Muslim possesses an executive power of the law of God". Although some Islamic jurists were nervous enough about the principle to insist that the state must enjoy a full monopoly of violence, the great majority allowed ordinary believers to enforce certain laws.

Where did this come from? Cook considers pre-Islamic Arab attitudes and the Roman idea of the correctio fraterna , which entered Christianity as the "fraternal correction" still acknowledged by Catholics today, but suggests that the clearest antecedent may be found in Jewish practice. Some rabbis held that Jerusalem was destroyed because the Jews "did not rebuke one another"; and similar language is present in the Koran. Nonetheless, the link is unproven; and Cook, apparently confident about the reliability of the early Arabic sources, suggests that we must simply add the duty to the list of Islam's novelties.

Geographically, Cook's survey does not attempt completeness. Arabic, Turkish and Persian literatures are used, but there is no coverage of the application of the duty in Hausaland, or as a theme in the anti-colonial struggle in Sumatra. Chronologically, however, he brings the story up to date. There are precious insights here into the transformation of religion in revolutionary Iran, where dissidents and the state invoke the principle to condemn each other. Women are increasingly involved, as are non-state organisations, and the traditional list of suspect practices has been enriched by the addition of Zionism and neo-colonialism.

Of immediate contemporary concern, too, is Cook's recognition that hardline readings of the duty underlie much modern Muslim extremism. The regimes have discredited themselves by opening the doors to satellite dishes and American troops, and the zealots react by reprinting medieval tracts on commanding the good. The duty offers one reason why "no other major religious tradition has lent itself to revival as a political ideology - and not just a political identity - in the modern world". The duty offers few clues about the necessary form of Muslim political process; it is a two-edged sword, allowing believers to become democrats or Taliban militants.

We cannot understand our world without understanding Islam; and Cook's text, with its respect for the intellectual diversity and sophistication of a tradition that is being invoked, correctly or aberrantly, by activists from Afghanistan to Algeria, will supply an invaluable historical foundation for any attempt to understand Muslim society.

Tim Winter is lecturer in Islamic studies, University of Cambridge.