How do the Houyhnhnms test Gulliver to determine his nature?

The Houyhnhnms assess Gulliver's nature primarily by observing how he interacts with Yahoos, who are brutish, human-like creatures. This interaction serves as a critical litmus test for determining Gulliver's character in relation to the more civilized and rational qualities embodied by the Houyhnhnms. The Houyhnhnms are known for their intelligence and reason, so offering Gulliver a chance to engage with Yahoos helps them evaluate whether he shares certain attributes with these uncivilized beings or if he aligns more closely with their own ideals.

While testing Gulliver through food could provide insights into his physical preferences, the true measure of his nature is better understood through his reactions and behaviors when faced with the Yahoos. This interaction highlights a significant theme in the narrative regarding the nature of humanity and civilization.