Psychologists referring to St Thomas Aquinas (1225-1274) in the context of hallucinations have neither accurately portrayed his conceptions of such experiences nor critically examined his alleged personal experience of them. This paper first examines Aquinas' conception of what are termed "hallucinations." It is shown that he allowed both natural and supernatural explanations for such experiences, with both accounts acknowledging an underlying physical cause. Contrasting explanations for his alleged personal experience of hallucinations are examined, including a new "Chestertonian" interpretation. Critiques of the use of his canonisation documents as factual evidence are also considered. It is concluded that Aquinas had a sophisticated understanding of hallucinations, and although it is fundamentally unknowable whether he actually experienced a hallucination, nevertheless, the approach we take to understanding his experience is of importance. The implications of Aquinas for interdisciplinary dialogue between theology and psychology in the field of hallucinations are then examined.