Mr. John Biddle was the Captain of the Explorer, the ship that happened to find Gulliver on its return from Japan by the North and South Seas.
Of course, they were all very curious to know how Gulliver’s ship was wrecked, how did he manage to save himself, and how did he spend those years away from civilisation.
When Gulliver told them candidly all that has happened to him, they took him for a lunatic, of course, and attributed his madness to the long years of solitude.
In order to prove his sanity, Gulliver produced his herd and the rest of his Lilliputian souvenirs.
Apparently, Captain Biddle saw the commercial possibilities of conquering Lilliput, and wasted no time in tracing Gulliver and inviting him to join this business venture.