Your make a good point  Let me first address it from the perspective of Marshall Hodgson  Civilization is a somewhat arbitrary term often narrowly defined by language  This obviously has its limits  On the other hand the more broadly you define a civilization the more subgroupings it must include  Eventually we come to see civilizations as groupings useful for the purpose at hand  And ultimately civilizations like religions are selfdefined  Hodgson considers the lands of Islam to constitute a world civilization  He calls Islam a world civilization in the title of his book The Venture of Islam Conscience and History in a World Civilization  For another perspective I can turn to H Patrick Glenn in Legal Traditions of the World speaking on tradition and identity  Not all is arbitrary in the definition of a tradition or civilization  Glenn quotes Fernand Braudel who speaks of an underlying structure of civilization those elements of a civilization which could not be changed without a fundamental change or more probably disappearance of the tradition itself  Applying this criteria to Islamic civilization one immediately understands that structure to be the religion of Islam  To summarize looking at the lands of Islam with its concept of Umma the shared traditions the shared Arabic religious and often secular language and indeed the shared religion of Islam it seems logical to fall in with those who perceive the outline of a world Islamic civilization