Ayre et al ^{Reference Ayre, Owen and Moran1} attempt to support the usefulness of the assessment of capacity in borderline personality disorder by validating it as a diagnostic construct. This is a flawed approach for several reasons.

1. • People with a diagnosis can move in and out of states of competence as much as those of us without a diagnosis; what would the Mental Capacity Act (MCA) have to say about a newly bereaved mother with suicidal thoughts (other than require us to invent a disorder of mind for her)?

2. • Acute presentations frequently preclude the ability to make a diagnosis; states of distress, psychoactive substances and lack of information all cloud our ability to make clear assessments; assessors vary in experience and competence and judgements about diagnosis vary among even those with comparable experience and skill.

3. • Because the MCA and popular culture privilege the place of diagnosis in determinations of responsibility, clinicians are led to first make a diagnosis and then, secondarily, make a determination of capacity – this results in predetermining capacity judgements based on diagnosis.

4. • Comorbidity is a frequent finding in personality disorder; however, the presence of Axis I disorder has the effect of ‘trumping’ the Axis II in the minds of professionals, the public and – crucially – patients, colouring patients' expectations of their ability to assume personal responsibility.

5. • Finally, there is an issue of tautology; capacity could not be impaired without some impairment of the individual's cognitive, perceptual or emotional state – the very abnormalities which indicate disorder of mind; the absence of capacity is thus sufficient to denote a disorder of mind and the requirement to name this directs clinicians to assign a diagnosis and attribute the incapacity to the diagnosis, rather than the aspect of function which impairs capacity.

We should be arguing to dissociate mental capacity from disorder of mind and, instead, deepen our thinking about the application of capacity judgements in clinical situations.