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 self defined 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