  Psillos (2001) refers to the “upward path” to structural realism as beginning with empiricist epistemological principles and arriving at structural knowledge of the external world. The “downward” path is to arrive at structural realism by weakening standard scientific realism as suggested by Worrall. Both paths are criticized by Psillos. Russell (1927) was led along the upward path by three epistemological principles: firstly, the claim that we only have direct access to our percepts (Ayer's ‘egocentric predicament’); secondly, the principle that different effects have different causes (which is called the Helmholtz-Weyl Principle by Psillos); and thirdly, that the relations between percepts have the same logico-mathematical structure as the relations between their causes. This led him to the claim that science can only describe the world up to isomorphism, and hence to (3) above since according to him we know only the (second-order) isomorphism class of the structure of the world and not the (first-order) structure itself. Russell's upward path is defended by Votsis (2005).