A good understanding of the nature of a property requires knowing whether that property is relational or intrinsic. Gabriel Segal's concern is whether certain psychological properties—specifically, those that make up what might be called the "cognitive content" of psychological states—are relational or intrinsic. He claims that content supervenes on microstructure, that is, if two beings are identical with respect to their microstructural properties, then they must be identical with respect to their cognitive contents.
Segal's thesis, a version of internalism, is that being in a state with a specific cognitive content does not essentially involve standing in any real relation to anything external. He uses the fact that content locally supervenes on microstructure to argue for the intrinsicness of content. Cognitive content is fully determined by intrinsic, microstructural properties: duplicate a subject in respect to those properties and you duplicate their cognitive contents.
The book, written in a clear, engaging style, contains four chapters. The first two argue against the two leading externalist theories. Chapter 3 rejects popular theories that endorse two kinds of content: "narrow" content, which is locally supervenient, and "broad" content, which is not. Chapter 4 defends a radical alternative version of internalism, arguing that narrow content is a variety of ordinary representation, that is, that narrow content is all there is to content. In defending internalism, Segal does not claim to defend a general philosophical theory of content. At this stage, he suggests, it should suffice to cast reasonable doubt on externalism, to motivate internalism, and to provide reasons to believe that good psychology is, or could be, internalist.