a context of belief, desire, or another mental state of a person which includes propositions that are taken to be true in that context, and entities that are taken to exist in that context. Example: in Harry thinks that a mountain lion is in the pine tree, the proposition that there is a mountain lion in the pine tree, and the existence of that particular mountain lion, is in the mental space of Harry's beliefs. (Sections 17.3.1, 18.2.2)