  Worrall's paper has been widely cited and has spawned an extensive literature in which various varieties of structural realism are advocated. These contemporary debates recapitulate the work of some of the greatest philosophers of science. Worrall says he found his structural realism in Henri Poincaré (1905, 1906) whose structuralism was combined with neo-Kantian views about the nature of arithmetic and group theory, and with conventionalism about the geometry of space and time. (The prevalence of Kantian themes in the literature on structural realism is discussed further below; for more on Poincaré see Giedymin 1982, Gower 2000 and Zahar 1994, 2001.) Ernan McMullin (1990) argues that Pierre Duhem was a realist about the relations found in laws but not about explanations in terms of an ontology. According to Worrall (1989), Barry Gower (2000) and Elie Zahar (2001), Duhem too was a kind of structural realist, though there are passages in Duhem that more readily lend themselves to an instrumentalist interpretation. Gower's (2000) historical survey of structural realism also discusses how structuralism figures in the thought of Ernst Cassirer, Moritz Schlick, Rudolf Carnap and Bertrand Russell. Stathis Psillos (1999) has explored the connections between structuralism and the Ramsey-sentence approach to scientific theory as it figured in the development of Carnap's philosophy from logical positivism to ontologically relativist empiricism. Other important pioneers of structuralism about science include Arthur Eddington (see French 2003), Grover Maxwell (see Ladyman 1998 and 3.1 below) and Hermann Weyl (see Ryckman 2005).