How should the mental element be defined in the legal standards governing a person’s ‘sectioning’ or placement under the Mental Health Act (MHA)? This article considers how this mental element is defined in many MHAs in Australasia: via a statutory list of disorders of mental function said to ‘characterise’ the necessary state of mind. This article assesses the assumptions behind the adoption of this approach. It discusses the views of several English law reform committees that have explored how the mental element should be defined. It examines the philosophy of psychiatry, expounded clearly by Aubrey Lewis, that lies behind the Australasian approach, one that emphasises the need to identify mental disturbance by reference to disorders of ‘part-function of the mind’, not by reference to behaviour alone. It considers how the Australasian statutes address the question of personality disorder’s covered by the Act. In conclusion, it endorses cautiously the Australasian approach, principally on the ground that it may contribute positively to the conduct of review proceedings for compulsory patients under the Act. It may concentrate the attention of tribunals on particular features of the patient’s mental state, on how those features are linked to associated dangers or risks, and on how the presence of those features may justify placing decisions about the patient’s treatment in others’ hands. Throughout, comparisons are made with the manner in which the mental element has been defined in mental health legislation for England and Wales.