 Like earlier direct realists, strong representationalists tend to be naturalists or physicalists. Therefore these theories usually come with a naturalist account of how these states (thought of as realized in the central nervous system) acquire their representational content. The most common account is either an ideal causal co-variation theory (informational semantics — see Dretske 1981, Fodor 1987, Aydede and Güzeldere 2005) or a teleological psychosemantics (see Millikan 1984, Papineau 1987) or both (Dretske 1988, 1995; Tye 1996a). These are externalist  theories.[16]