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Structural realism is a philosophy of science that contends scientific theories provide accurate descriptions of the structural features of reality while remaining agnostic about its underlying metaphysical nature. This viewpoint emphasizes the persistence of mathematical and relational frameworks across theoretical change, distinguishing it from both naive realism and anti-realist positions. Early formulations by John Worrall in the 1980s drew on historical case studies to argue that even discarded scientific theories preserve their empirical and structural insights. Later, Ladyman and Ross advanced ontic structural realism in *Every Thing Must Go* (2007), positing that structures are fundamental to reality itself, not merely epistemic constructs. James Ladyman has further refined these ideas, grounding structural realism in contemporary physics and metaphysics. Advocates of this viewpoint hold it offers a robust framework for understanding scientific progress, theory evaluation, and the limits of human knowledge.

John Worrall's epistemic structural realism, articulated in his 1989 essay “Structural Realism: The Best of Both Worlds?”, posits that scientific theories accurately describe the structural features of reality while remaining agnostic about the intrinsic properties of unobservable entities. Worrall argues that even when scientific theories are discarded or revised, their mathematical and relational frameworks often persist, suggesting that these structures capture something objectively true about the world. This view avoids commitment to unobservable metaphysical posits by focusing on the empirical success of theoretical relations. Proponents contend it reconciles realism with the historical instability of scientific knowledge, as structural continuity across theories provides a stable basis for scientific progress.

Ontic structural realism, developed by James Ladyman and Don Ross in *Every Thing Must Go* (2007), goes further by asserting that mathematical structures are ontologically fundamental to reality itself. This position holds that the relational patterns described by successful theories reflect objective features of the world, independent of human observation or measurement. Advocates argue that contemporary physics, particularly quantum mechanics and general relativity, provides evidence for this view, as these theories prioritize structural relations over substantive descriptions of matter. Ontic structuralists maintain that structures are prior to objects, meaning reality is fundamentally composed of relational frameworks rather than underlying entities with intrinsic properties.

Steven French's eliminativist ontic structural realism, developed across a series of works culminating in *The Structure of the World: Metaphysics and Representation* (2014), holds that objects should be eliminated from our fundamental ontology in favor of structures alone. French argues that the indistinguishability of quantum particles — their failure to behave as individual, countable objects in the way classical physics assumes — reveals that what science actually describes is a network of relations without underlying object-like bearers. On this view, entities such as electrons are not objects that stand in structural relations; they are nothing over and above those relations. Proponents hold that this eliminativist stance provides the most metaphysically parsimonious reading of quantum theory, avoiding the need to posit mysterious intrinsic properties or haecceitistic individuals that play no explanatory role.

Michael Esfeld's non-substantialist structural realism, presented in his 2013 paper “Ontic Structural Realism and the Interpretation of Quantum Mechanics,” rejects the notion of substantive matter entirely. Esfeld holds that physical objects are nothing but value assignments within a relational structure, derived from quantum theory's mathematical formalism. This position avoids metaphysical excess by eliminating references to intrinsic properties, instead grounding reality in spacetime structures and their dynamic relations. Proponents argue it provides a parsimonious account of quantum phenomena without invoking unobservable entities or hidden variables.

A central argument across these variants is that scientific theories succeed by capturing structural relations, even when underlying mechanisms remain elusive. This explanatory power — evidenced by successful predictions — validates the reality of theoretical structures without requiring ontological commitment to unobservables. Critics have pressed two principal objections. Stathis Psillos argues that structures cannot be coherently defined without presupposing the very objects and intrinsic properties structural realists seek to eliminate or bracket, making the position circular. Separately, philosophers including John Earman have raised empirical underdetermination concerns, contending that multiple incompatible structures can fit the same data, leaving structural realists without a principled basis for preferring one relational framework over another.

John Worrall initially articulated epistemic structural realism in his 1989 paper “Structural Realism: The Best of Both Worlds?”, where he argued that scientific progress preserves the structural core of theories despite empirical revisions.

James Ladyman and Don Ross advanced ontic structural realism in their 2007 book *Every Thing Must Go*, contending that structures are fundamental to reality, not merely epistemological constructs.

Steven French defends an eliminativist variant of ontic structural realism, arguing in *The Structure of the World: Metaphysics and Representation* (2014) that quantum mechanics warrants the removal of objects from fundamental ontology in favor of pure structural relations.

Michael Esfeld defends a non-substantialist variant of structural realism, particularly in quantum mechanics, as outlined in his 2013 paper “Ontic Structural Realism and the Interpretation of Quantum Mechanics.”

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1. John Worrall, “Structural Realism: The Best of Both Worlds?”, *Dialectica* 43, no. 1–2 (1989): 99–124. 2. James Ladyman and Don Ross, *Every Thing Must Go: Metaphysical Naturalism in the Age of Science* (Oxford: Oxford University Press, 2007). 3. Michael Esfeld, “Ontic Structural Realism and the Interpretation of Quantum Mechanics,” *European Journal for Philosophy of Science* 3, no. 1 (2013): 19–32. 4. Steven French, *The Structure of the World: Metaphysics and Representation* (Oxford: Oxford University Press, 2014).