political concept of Gulliver's country of Houyhnhnms could not understand, why?

When Gulliver explains the relationship of horses and humans back in England, the horses cannot believe that humans would be able to control creatures that are so much stronger than they are. Gulliver goes on to explains that horses are tamed beginning at a very young age.

Source(s): http://www.gradesaver.com/gullivers-travels/study-guide/section6/