Mindfulness exercises are presented as being compatible with almost any spiritual, religious or philosophical beliefs. In this paper, we argue that they in fact involve imagining and conceptualising rather striking and controversial claims about the self, and the self’s relationship to thoughts and feelings. For this reason, practising mindfulness exercises is likely to be in tension with many people’s core beliefs and values, a tension that should be treated as a downside of therapeutic interventions involving mindfulness exercises, not unlike a side effect. Clients ought to be informed of these metaphysical aspects of the exercises, and mental health providers ought to take them into account in assessing which course of treatment to recommend. Given these concerns, the casual way in which mindfulness exercises are presently distributed by mental health providers to the general public is inappropriate.