 The simplest case is where the questioner requires the responder to commit to a view on whether some primary concept is instantiated in the world. For example, “Is there or is there not a vacuum?” More complicated, because it combines two concepts, is a question like “Is the sky spherical or not?” More bizarre, “Stone and human, which of the two is animal?” ([Debate] 43.16–20) When the responder has made a commitment, the questioner must devise a syllogism whose conclusion contradicts the commitment, and then attempt to get the responder to commit to the premises of this syllogism and to its validity. The responder is allowed to challenge on any of these points, whereupon the two participants swap roles. 