Structural Realism sketchy intro

There are a variety of flavours of Structural Realism; the main division is between Ontic and Epistemic versions, which say respectively that "structure" is all that exists, or at least all that we can have knowledge of, in the world. Any SR theory has to give an acount of what structure is, and what knowledge of it we can come to have (if any), alongside dealing with whatever questions the theory is proposed to answer in the first place.

ESR itself divides into two major varieties, which Stathis Psillos has dubbed the "upward" and "downward" paths. The downward path, as championed by Worrall (with foreshadowing by Poincare), starts with the problem of theory change in science, and wonders just what it is that science is getting right even when its old theories are shown to be wrong. The anti-realist says "nothing", and is confronted with the then seemingly inexplicable successes of scientific prediction; the full-on realist says "everything" (at least, the most naive kind does), and then has some explaining to do whenever a field of science seems to undergo a radical shift. Worrall aims to steer a middle course, by finding something substantial that is retained even through radical theory change. This something is "structure" - specifically, the mathematical (or otherwise abstract) structures onto which interpretations are pasted. His standard example is Fresnel's account of light as transverse waves in a luminiferous aether: throw out the aether and bring in electromagnetic fields, and Maxwell's acount of light looks very similar in many important respects. The wave equations and other key features are preserved from Fresnel even while the basic ontology is overturned.

The upward path, which is associated with Russell, deals with questions about the correspondence between our perceptions and the outside world. In brief, it proposes that there are objects with relations between them in the world, and that our perceptions and the relations between them are in some way a reflection of those outside. Exactly what the connection might be, and how strong it is, has been debated by (for example) Psillos and Ioannis Votsis.