But man differs from other animals in one very important respect, and that is that he has some desires which are, so to speak, infinite, which can never be fully gratified, and which would keep him restless even in Paradise. The boa constrictor, when he has had an adequate meal, goes to sleep, and does not wake until he needs another meal. Human beings, for the most part, are not like this. When the Arabs, who had been used to living sparingly on a few dates, acquired the riches of the Eastern Roman Empire, and dwelt in palaces of almost unbelievable luxury, they did not, on that account, become inactive. Hunger could no longer be a motive, for Greek slaves supplied them with exquisite viands at the slightest nod. But other desires kept them active: four in particular, which we can label acquisitiveness, rivalry, vanity, and love of power.