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The quantum ontology as a domain without objects

Bruno José Borge, Olímpia Lombardi

Published Jan 1, 2026• 0 citations
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Abstract

Standard quantum mechanics poses serious challenges to some principles deeply entrenched in traditional metaphysics. In particular, the theory defies the ontological category of individual object through three of its features: contextuality, non-separability, and indistinguishability. The difficulties have prompted the development of non-traditional ontological frameworks intended to account for the quantum domain. Some approaches advocate replacing the classical conception of a realm of objects endowed with properties by an ontology that either dispenses with the category of object altogether or treats it as a derivative notion. The Modal-Hamiltonian Interpretation (MHI) of quantum mechanics and Ontic Structural Realism (OSR) exemplify this position. MHI proposes an ontology in which quantum systems are non-objectual bundles of properties linked by definite physical relations. The OSR defends the ontological primacy of structures, within which objects are construed as nodes of relations. The aim of this article is to compare these two ontological approaches, highlighting both their points of convergence and their divergences, particularly in light of the ontological challenges posed by quantum mechanics. With this aim, the article is structured as follows. Section 2 outlines the ontology of MHI, introducing the distinction between type-properties and case-properties, characterizing quantum systems as non-objectual bundles, and analyzing the role of symmetry groups in providing physical content to the theory and governing actualization. Section 3 turns to OSR, examining its motivations, its distinction between physical and mathematical structures, and the modal character of structures. Section 4 offers a systematic comparison of the two approaches, considering their respective motivations and scope, their shared commitment to an object-free ontology, the role played by symmetry groups within each framework, and their contrasting interpretations of modality. Finally, Section 5 presents some concluding remarks.

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Standard quantum mechanics poses serious challenges to some principles deeply entrenched in traditional metaphysics. In particular, the theory defies the ontological category of individual object through three of its features: contextuality, non-separability, and indistinguishability. The difficulties have prompted the development of non-traditional ontological frameworks intended to account for the quantum domain. Some approaches advocate replacing the classical conception of a realm of objects endowed with properties by an ontology that either dispenses with the category of object altogether or treats it as a derivative notion. The Modal-Hamiltonian Interpretation (MHI) of quantum mechanics and Ontic Structural Realism (OSR) exemplify this position. MHI proposes an ontology in which quantum systems are non-objectual bundles of properties linked by definite physical relations. The OSR defends the ontological primacy of structures, within which objects are construed as nodes of relations. The aim of this article is to compare these two ontological approaches, highlighting both their points of convergence and their divergences, particularly in light of the ontological challenges posed by quantum mechanics. With this aim, the article is structured as follows. Section 2 outlines the ontology of MHI, introducing the distinction between type-properties and case-properties, characterizing quantum systems as non-objectual bundles, and analyzing the role of symmetry groups in providing physical content to the theory and governing actualization. Section 3 turns to OSR, examining its motivations, its distinction between physical and mathematical structures, and the modal character of structures. Section 4 offers a systematic comparison of the two approaches, considering their respective motivations and scope, their shared commitment to an object-free ontology, the role played by symmetry groups within each framework, and their contrasting interpretations of modality. Finally, Section 5 presents some concluding remarks.
Abstract In Chapter 4, the MHI was introduced in terms of interpretative postulates that define the ontological reference of each physical term of quantum mechanics in its standard formulation. On the basis of that presentation, an ontology of properties, framed in the algebraic formalism of quantum mechanics, was proposed. The aim of this chapter is to set out in detail the structure of the ontology proposed by the MHI, now with the technical metaphysical resources developed in the previous chapter. In particular, the nature of the properties that constitute the fundamental ontological items is discussed, and it is argued that quantum systems are not objects at all, but mere collections of properties that do not retain their identity either synchronically or diachronically. More precisely, quantum systems turn out to be non-objectual bundles of properties.
Abstract In the previous chapter, the structure of the ontology proposed by the MHI was introduced. In this ontology, properties are the fundamental items and quantum systems are non-objectual bundles of properties. However, as is well known, metaphysics is underdetermined by physics; in particular, quantum mechanics is compatible with different ontological pictures. Thus, arguing in favor of a certain quantum ontology over others requires showing how fruitful it is in the task of offering reasonable solutions to interpretative problems. This chapter is devoted to show the advantages of the proposed picture for dealing with the ontological challenges of quantum mechanics: contextuality, non-separability, and indistinguishability. Finally, the top-down approach presented in Chapter 8 is revisited, showing how it acquires an ontological significance from the perspective of the ontology of properties proposed by the MHI.
Abstract The issue about the nature of quantum systems usually hinges on the category of individual: the question is whether this category can be applied to understand the peculiarities of quantum behavior. However, some metaphysical subtleties are commonly overlooked in this discussion. What is the difference between the notions of object and of individual? What metaphysical principles are quantum systems supposed to fulfill? What ontological kinds of properties are involved in the quantum domain? These questions should be taken into account when the issue at stake is the ontological nature of quantum systems and, more generally, the structure of the quantum realm. In order to address these and other metaphysical questions, different ontological distinctions that are scarcely taken into account in the debate are brought to the fore in this chapter. In light of these distinctions, the ontological challenges that quantum mechanics poses to the ontological picture presupposed by traditional metaphysics will be discussed in the following chapter.
In correspondence with the modal interpretations of quantum mechanics, Lombardi and Castagnino (2008) and da Costa, Lombardi and Lastiri (2013) have proposed an ontology of bundles of possible intrinsic properties for quantum mechanics, focused mainly on the issues of quantum contextuality and indistinguishability. Corresponding to the interpretation known as "relational quantum mechanics" (Rovelli, 1996), two ontologies based on relations have been proposed. Laura Candiotto (2017) advocates for a radical metaphysics of relations, considered by the author as an instance of the so-called "ontic structural realism". Alternatively, Andrea Oldofredi (2021) proposes an ontology of bundles of relations, considered by the author as an instance of the so-called "moderate structural realism". Since the 1980s, the topic of quantum entanglement has led several authors (Teller, Healey, Howard, Esfeld) to defend that quantum mechanics involves a form of holism. It has been argued that the holism of quantum mechanics supports the metaphysical thesis known as "priority monism" (Schaffer, 2010a). In this article, an ontology of quantum mechanics is proposed in which the properties of quantum systems have both a modal and a relational character. As a result, a holistic ontology of quantum mechanics is obtained, which is an instance of priority monism and moderate structural realism.
The aim of this paper is to introduce a new member of the family of the modal interpretations of quantum mechanics. In this modal-Hamiltonian interpretation, the Hamiltonian of the quantum system plays a decisive role in the property-ascription rule that selects the definite-valued observables whose possible values become actual. We show that this interpretation is effective for solving the measurement problem, both in its ideal and its non-ideal versions, and we argue for the physical relevance of the property-ascription rule by applying it to well-known physical situations. Moreover, we explain how this interpretation supplies a description of the elemental categories of the ontology referred to by the theory, where quantum systems turn out to be bundles of possible properties.
The paper sides with the primitive ontology approach to quantum physics, as exemplified notably in Bohmian mechanics. It argues that the objects of the primitive ontology of quantum physics are structurally individuated by the spatial relations in which they stand. They neither have a primitive thisness (haecceity) nor intrinsic properties (such as mass or charge), but only stand in spatial relations. These relations make it that they are physical objects. Furthermore, in virtue of these relations, they are individuals, being absolutely discernible. Ontic structural realism thus plays a twofold role in the philosophy of quantum physics: it comes in not only at the level of the dynamics (structures of entanglement), but already at the level of the objects (primitive ontology) that instantiate the structures of entanglement, these objects being individuated by spatial relations.