  As mentioned above, Poincaré's structuralism had a Kantian flavour. In particular, he thought that the unobservable entities postulated by scientific theories were Kant's noumena or things in themselves. He revised Kant's view by arguing that the latter can be known indirectly rather than not at all because it is possible to know the relations into which they enter. Poincaré followed the upward path to structural realism, beginning with the neo-Kantian goal of recovering the objective or intersubjective world from the world from the subjective world of private sense impressions: “what we call objective reality is… what is common to many thinking beings and could be common to all; … the harmony of mathematical laws” (1906, 14). However, he also followed the downward path to structural realism arguing that the history of science can be seen as cumulative at the level of relations rather than objects. For example, between Carnot's and Clausius' thermodynamics the ontology changes but the Second Law of Thermodynamics is preserved. While Worrall never directly endorses the Kantian aspect of Poincaré's thought, Zahar's structural realism is explicitly a form of Kantian transcendental idealism according to which science can never tell us more than the structure of the noumenal world; the nature of the entities and properties of which it consists are epistemically inaccessible to us (as in (2) above). Michaela Massimi (2011) develops a neo-Kantian perspective on structural realism.