An ontological causal relation is a quantified relation between certain interactions and changes in corresponding properties. Key ideas in physics, such as Newton’s second law and the first law of thermodynamics, are representative examples of these relations. In connection with the teaching and learning of these relations, this study investigated three issues: the appropriate view concerning ontological category, the role and status of ontological causal relations, and university students’ understanding of the role and status of these relations. Concerning the issue (...) of proper ontology, this study suggests an alternative view that distinguishes between interaction and property at the macroscopic level, in contrast to Chi and colleagues’ influential view. Concerning the role and status of the relations, we conclude that fundamental ontological causal relations should be regarded as knowledge at the core of relevant physics theories. However, upon analysis of participants’ responses, this study finds that university students’ views on the status of the heat capacity relation and Newton’s second law are quite different. Several possible educational implications of these results are discussed. (shrink)