My thoughts here. The Arabs fail to capture Constantinople or to hold Rome or their gains in France, and start to lose ground to the Byzantines as well. Much discourse on Arabic learning. Gibbon concludes that three reason they failed to make much headway after the eighth century were:

• "When the Arabian conquerors had spread themselves over the East, and were mingled with the servile crowds of Persia, Syria, and Egypt, they insensibly lost the freeborn and martial virtues of the desert."
• "The sect of the Carmathians [Qarmatians] may be considered as the second visible cause of the decline and fall of the empire of the caliphs."
• "The third and most obvious cause was the weight and magnitude of the empire itself."

Also some reflections of mine on happiness, and on science and learning.