Look two paragraphs further up to where he’s setting the scene for this thought experiment:
There are all manner of signs and portents indicating whether a box contains a diamond; but I have no sign which I know to be perfectly reliable. There is a blue stamp on one box, for example, and I know that boxes which contain diamonds are more likely than empty boxes to show a blue stamp. Or one box has a shiny surface, and I have a suspicion—I am not sure—that no diamond-containing box is ever shiny.
And it’s in that context that he postulates a “clever arguer” who tries to persuade him by listing (true) facts like “box B shows a blue stamp”.
Look two paragraphs further up to where he’s setting the scene for this thought experiment:
And it’s in that context that he postulates a “clever arguer” who tries to persuade him by listing (true) facts like “box B shows a blue stamp”.