Holistic thinking is so diverse that practically every theorist can claim holistic credentials. The challenge for nursing, then, is not whether holism but which holism? As a practice-centred discipline, nursing gives a central role to whole-person holism. In so far as nursing approximates medicine, nursing imports organismic holism. The three holisms are, however, incompatible because the concepts of person, system and organism contrast in complex and interesting ways. Discussion of the contrasts reveals points of metaphysical, scientific and ethical incompatibility. A construct based on whole-person holism, but incorporating a partial synthesis of all three, reveals both promise and challenge for a unified holistic view in nursing.