What position on dualism does medicine require? Our understanding of that ques- tion has been dictated by holism, as defined by the biopsychosocial model, since the late twentieth century. Unfortunately, holism was characterized at the start with con- fused definitions of ‘dualism’ and ‘reductionism’, and that problem has led to a deep, unrecognized conceptual split in the medical professions. Some insist that holism is a nonreductionist approach that aligns with some form of dualism, while others insist it’s a reductionist view (...) that sets out to eradicate dualism. It’s important to consider each version. Nonreductive holism is philosophically consistent and clinically unprob- lematic. Reductive holism, however, is conceptually incoherent—yet it is the basis for the common idea that the boundary between medical and mental health disorders must be vague. When we trace that idea through to its implementation in medical practice, we find evidence that it compromises the safety of patient care in the large portion of cases where clinicians grapple with diagnosis at the boundary between psychiatry and medicine. Having established that medicine must embrace some form of nonreduc- tionism, I argue that Chalmers’ naturalistic dualism is a stronger prima facie candidate than the nonreductive alternatives. Regardless of which form of nonreductionism we prefer, some philosophical corrections are needed to give medicine a safe and coherent foundation. (shrink)