有读者问：
(1)I also saw that the camel must be blind in the right eye.
这句话中为什么用must而不用had to?
must在此表示“推测”，它和意谓“必须”的must一样，也可用在间接引语中。这里的that从句亦可看做是间接引语。但该句的must又和意谓“必须”的must不一样，它在间接引语中不可改作had to，再如：
(2)He thought that I must have lived here for a long time.
表示“推测”的must亦可用在一般的表过去的叙述中，如：
(3)He managed to drag himself home where he lay down on the iron cot and remained, panting and gasping. He must have dozed off because he imagined that he was in his hometown.
