 Chalmers develops this line of thought with the help of the following apparatus. A thought is said to be epistemically possible if it cannot be ruled out a priori, i.e. if its negation cannot be conclusively established without any appeal to experience. Such a thought corresponds to an epistemic possibility, a way the world could be for all one can tell a priori. A scenario is then defined to be a maximally specific epistemic possibility, an epistemic possibility with no detail left unspecified. Epistemic space is the set of all such scenarios. Any thought carves out a particular region of epistemic space by endorsing some scenarios and excluding others. A thought endorses a scenario when, if we accept that the scenario is actual, we should accept the thought as true. For instance, if we accept as actual a scenario in which the liquid that falls from the skies and fills the lakes is XYZ, we should accept as true the thought that water is XYZ. We can then think of the narrow content of a thought as constituted by the way the thought divides epistemic space into those scenarios it endorses and those it excludes. More specifically, we can think of the narrow content of a thought as a function from scenarios to truth values, or (equivalently if we have only two truth values) simply as a set of scenarios, namely those endorsed by the thought. (This paragraph closely follows Chalmers 2002, p. 610. Chalmers gives related but somewhat more detailed expositions in Chalmers 2003, especially pp. 47, 54, and in Chalmers 2006, especially pp. 76ff. Note that a thought endorses a scenario iff the scenario verifies the thought: Chalmers 2006 uses the latter terminology but not the former.)