  Structural realism is often characterised as the view that scientific theories tell us only about the form or structure of the unobservable world and not about its nature. This leaves open the question as to whether the natures of things are posited to be unknowable for some reason or eliminated altogether. Hence, Ladyman (1998) raised the question as to whether Worrall's structural realism is intended as a metaphysical or epistemological modification of standard scientific realism. Worrall's paper is ambiguous in this respect. That he has in mind only an epistemic constraint on realism—commitment to the structure of our best scientific theories but agnosticism about the rest of the content—is suggested by his citation of Poincaré who talks of the redundant theories of the past capturing the “true relations” between the “real objects which Nature will hide forever from our eyes” (1905, 161). So one way of thinking about structural realism is as an epistemological modification of scientific realism to the effect that we only believe what scientific theories tell us about the relations entered into by unobservable objects, and suspend judgement as to the nature of the latter. (ESR is called ‘restrictive structural realism’ by Psillos 2001.) There are various forms this might take. (See French and Ladyman 2011.)