Marty's Dad says a witness is

"Somebody who sees something happen and signs that it's true."

Marty's Ma says a witness is

"Somebody who knows the Lord Jesus and don't mind tellin' about it."

When Marty asks his parents for the definition of a witness, the answer he is looking for is the one he gets from his father. Marty has made Judd Travers sign an agreement saying that Shiloh will belong to Marty in exchange for twenty hours of work, but halfway through the twenty hours, Travers laughs at Marty and tells him the agreement is not binding because there were no witnesses to its signing. Marty doesn't know what a witness is exactly, so he asks his parents, and after they give their answers, he asks his Dad if it is true that in order for a bargain to be binding, there must be a witness. Marty's Dad concedes that he supposes that it is true, technically, and Marty is angry and discouraged for not having known that before. After considering his options, however, he decides there is nothing he can do about it but keep his end of the bargain and hope for the best. In doing so, Marty inadvertently performs the other kind of witnessing, showing Travers, who has experienced little love or kindness in his life, something about integrity and generosity, values taught by the Lord Jesus. In doing so, he wins the harsh man's respect and gets to keep Shiloh. Although he did not know to have the kind of witness his Dad speaks about, by witnessing the way his Ma describes, Marty ensures that Travers keeps his end of the bargain (Chapter 15).