In the context of definitional confusion, it might seem recklessly counter-productive to suggest that what we agree on in relation to subsidiarity is perhaps more problematic than what we disagree on. Nonetheless, the purpose of this article is to challenge the uncontroversial starting-point of much subsidiarity analysis: that subsidiarity allocates functions between 'higher' and 'lower levels'. Recent scholarship on subsidiarity in political philosophy expresses a frank concern about a de-contextualized and dogmatic use of those terms and reminds us that subsidiarity is premised on a pre-existing social ontology. In order to recognize subsidiarity's ontological commitments, this paper proposes an ontology-sensitive approach to subsidiarity, and the use of the terms primary units (in place of 'lower levels') and subsidiary units (in place of 'higher levels'). Discussion of how the ontology-sensitive approach works in practice, through the fictional example of the Good Fight Club, reveals four precepts which direct the interaction between primary and subsidiary units. Finally, these four precepts are used as benchmarks against which to appraise the European principle of subsidiarity.