The debate over the validity of psychiatric diagnoses remains a central, unresolved issue in psychological science. Although it is often taken to be an ontological matter, this paper argues that, in fact, the dispute stems from differences in epistemology. Two epistemological themes, the objective of science and the nature of causality, are discussed to provide examples of how commitments made at this level inevitably lead to differing ontological conclusions about whether diagnoses constitute valid clinical terms. Discussions that fail to address the deeper epistemological disagreements that underpin the opposing arguments will never fully capture the complexity of the dispute and hence are unlikely to lead to its resolution.