Continuing with my summary of the nature and sources of the shari‘a:
This is important because Islamic law covers a far wider range of actions than anything we know of as law in our own experience. After centuries of analysis and refinement, Muslim legal thinkers worked out a system in which all possible human acts were placed into one or another of five classes. Some were “obligatory” These actions must be done by every person. (Prayer might serve as a good example.) Others were merely “desirable.” It would be good to perform these acts, but a person would not be punished if he or she did not do so. While becoming a scholar of Islamic law is a good thing, for instance, a peasant will not be punished because he is not such a scholar. Nor, among educated people, will a doctor or an engineer be condemned simply because he or she has chosen a different but respectable career path. A third category comprises acts to which the law is “indifferent.” Whether a man or woman performs one or another of these acts is of no concern to Islam. (Should Mahmud work as an accountant, or should he seek to be a doctor? The law doesn’t care.) There are also actions that the law considers “objectionable.” People who act in such ways will not be punished, but it would be better if they avoided such actions. (Slurping one’s soup is not a moral offense and will not land anybody in Hell, but it is still not exactly admirable behavior.) Finally, there are actions like adultery and murder that are simply “prohibited.” This fifth category of actions consists of things that all people should avoid and for which, if they do not, they will be punished.
 Adultery no longer seems to fall under this category in Western law—although it once did, and although it seems to be a clear case of breach of contract at the very least.