Thanks for the comments Jack. I agree with much of what you said about Byzantium and think we may be saying the same thing about external enemies. As you say, after the initial conflict, Islam didn´t have any, but there were plenty in the West. And what is a succesive invader but a successful external enemy? So we seem to be saying the same thing about the contrast between Islam in the East and the invaders of the Western Empire. I also agree that the Spainish Moors received their civilisation from the East so I don´t think the damage to Spain by the Vandals and Goths effected them so much.
What I have learnt from Jared Diamond is to avoid arguments about the traits of specific cultures (such as the Romans being too practical to bother with philosophy) because you can pick holes in them and they are almost always ad hoc anyway. Besides, I am not sure I agree that the Romans had less interest in philosophy - they simply used Greek as the language for it. This only had consequences once Greek speakers were no longer available as teachers, as Boethius foresaw.