"A grand debate at the general assembly of the Houyhnhnms, and how it was determined. The learning of the Houyhnhnms. Their buildings. Their manner of burials. The defectiveness of their language."
The Houyhnhnms hold one of their four-year grand assemblies while Gulliver is there.
They go back to an old debate: whether Yahoos should be wiped off the face of the Earth.
On the side of "yes": they're disgusting and they have to be watched constantly to keep them from doing bad things.
Also, Yahoos are not native to Houyhnhnm Land: a man and a woman arrived one day, washed up on the shores of the island.
The Houyhnhnms caught and tamed their children.
The Master Horse speaks up to say, yes, it seems likely that these two original Yahoos came from over the sea, because the Master Horse has found one (Gulliver) who is a much better specimen of the Yahoo kind.
The Master Horse tells his fellows that, in Gulliver's land, Houyhnhnms are the servants and Yahoos are the rational animals.
The Master Horse also informs them about the human practice of castrating horses to make them less aggressive – why don't the Houyhnhnms try this method on young Yahoos of their own country?
This way, the Houyhnhnms could make the Yahoos more docile, which would mean they wouldn't need to kill them all.
The Houyhnhnms don't write anything down; they rely on oral records for their history.
They also don't have much in the way of astronomy, except to measure months and years.
They write beautiful poetry about friendship and in praise of their athletes.
Unless they have some kind of accident, they only die of old age, usually at around 70 or 75.
All of their words for something bad are connected to Yahoos, so a poorly built house is ynholmhmrohlnw Yahoo, and a stone that cuts their feet, ynlhmndwihlma Yahoo.