Located in a very low region of the brain, the RAS has the job of filtering all incoming stimuli and making the decision as to whether we attend or ignore something. The specific example he uses in his book is that of the Lincoln-Douglas debates in the 1800's which were literally read from paper and lasted for hours.
Postman notes that amazingly, the people stayed, listened and paid attention.
Today, I doubt we could expect to read any statement for 8 - 10 hours and have an audience of people stay in the room, let alone stay focused. Let me begin by having you think for a moment about attention. First, notice how your mind does not multitask - it has only one focus of attention at a time. You can think about last Christmas vacation. Now think about next week's schedule.