On Part IV of Gulliver’s Travels Johnathan Swift changes from fictional literature to non-fictional literature. To do so, he talks about the human nature in a first-person “The reader may please to observe that the following extract of the many conversations I had with my master, contains a summary of the most material points, which were discoursed at several times for above two years”.
So basically he changes from the main story of the book, which is the travels of Gulliver, to talk about human nature.