Robert R. Reilly: The first four caliphs remained on the Arabian peninsula. At first, they kept their troops quarantined outside the cities they had conquered so that Muslims wouldn't be contaminated by alien cultures and beliefs. After the founding of the Umayyad caliphate around 660, the center of the new empire moved to Damascus, and then later the Abbasids moved it to Baghdad. They couldn't maintain the quarantine, and they encountered peoples for whom philosophy had been second nature, as it was infused in Christian apologetics at the time.