Spiritism is widely accepted in Brazil and influences psychiatric practice, especially through religious-oriented hospitals. However, during the first half of the twentieth century it was considered an important cause of mental illness. This paper first reviews opinions on 'Spiritist madness', written by the most eminent psychiatrists of the time, and then discusses the epistemological factors that have contributed to the conflict between medicine and Spiritism. We critically examine the appropriateness of the methods used in the debates, and how this has led to inferences about associations and causal relationships.