Abstract: In this paper, we explore the ontological status of particles and fields in Quantum Field Theory (QFT). We analyze whether QFT can provide a reasonable foundation for particle ontology from the perspective of the state space. In free theories, the quantum states form a Fock space, supporting the view that particles and fields are equally fundamental. But in interacting theories, Haag’s theorem implies that although the state space of interacting theories retains a Fock space structure, it is not equivalent to the state space of free theories. The particle ontology is therefore a secondary interpretation. Under the perspective of effective field theory, the field ontology is relatively more fundamental than the particle ontology.

Key Words: Quantum Field Theory; Ontology; Fock representation; Haag’s theorem; Effective field theory

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