  Frank Jackson (1998), Rae Langton  (1998) and David Lewis (2009) also advocate views similar to ESR. Jackson refers to ‘Kantian physicalism’ (1998: 23–24), Langton to ‘Kantian Humility’, and Lewis to ‘Ramseyan Humility’. Peter Unger (2001) also argues that our knowledge of the world is purely structural and that qualia are the non-structural components of reality. Jackson argues that science only reveals the causal / relational properties of physical objects, and that “we know next to nothing about the intrinsic nature of the world. We know only its causal cum relational nature” (1998: 24). Langton argues that science only reveals the extrinsic properties of physical objects, and both then argue that their intrinsic natures, and hence the intrinsic nature of the world, are epistemically inaccessible. Jackson points out that this inference can be blocked if the natures of objects and their intrinsic properties are identified with their relational or extrinsic properties, but argues that this makes a mystery of what it is that stands in the causal relations. Lewis' structuralism is based on the centrality he gives to the Ramsey sentence reconstruction of scientific theories that is the subject of the next section.