Vagrancy is his first freedom. He does not even choose to go traipsing down the Mississippi with Jim, who just happens to be on Jackson's Island when Huck gets there. The book is one happening after another; Huck happens to fall in with a runaway slave instead of living by the book with Tom Sawyer. As Pap Finn chooses the mud, so Huck chooses the river. But didn't the river really choose him?
