["Journal of Agricultural Economics, EarlyView. ", "\nABSTRACT\nAgricultural and food policy is increasingly shaped by geopolitical instability, ecological constraints, technological disruption, political polarisation and declining trust in expertise. At the same time, calls for food system transformation have gained prominence as evidence accumulates on the environmental, health and social shortcomings of current food systems. Yet reform has frequently stalled despite extensive scientific analysis and broad recognition of the need for change. This paper examines why. After reviewing some traditional explanations for policy persistence, the paper introduces the concept of policy ontologies: underlying ways of understanding what the food system is, how it functions, what it is ultimately for and how change occurs. Three ideal‐type ontologies are identified: a productivity ontology, a power‐focused ontology and an ecological ontology. These ontologies shape the definition of policy problems and the selection of preferred policy responses, helping to explain why stakeholders often appear to talk past one another. As the Society begins its second century in a more turbulent and contested policy environment, the profession's central role remains the production of rigorous evidence. That contribution will be strengthened by a greater awareness of the different ways in which food systems are understood and of how these perspectives shape both policy debates and the interpretation of evidence. Recognising these underlying assumptions can help agricultural economists contribute more effectively to policy debate as honest brokers of policy alternatives.\n"]