 Vindicating this “golden triangle” constitutes a primary theoretical constraint for a rationalist interpretation of 2D semantics. A 2D semantics that meets this constraint would play a wide-ranging role in philosophy. It would account for core semantic roles associated with the Fregean notion of sense (Chalmers 2002b) and the traditional notion of a proposition (Chalmers 2011a). In addition, rationalist 2D semantics promises to define a versatile notion of narrow thought content suited to playing key explanatory and evaluative roles in commonsense psychology (Chalmers 2002c). Furthermore, the rationalist approach to meaning and modality underwrites a distinctive form of apriori reasoning about the nature of the actual world: