It is commonly thought that mental disorder is a valid concept only in so far as it is an extension of or continuous with the concept of physical disorder. A valid extension has to meet two criteria: determination and coherence. Essentialists meet these criteria through necessary and sufficient conditions for being a disorder. Two Wittgensteinian alternatives to essentialism are considered and assessed against the two criteria. These are the family resemblance approach and the secondary sense approach. Where the focus is (...) solely on the characteristics or attributes of things, both these approaches seem to fail to meet the criteria for valid extension. However, this focus on attributes is mistaken. The criteria for valid extension are met in the case of family resemblance by the pattern of characteristics associated with a concept, and by the limits of intelligibility of applying a concept. Secondary sense, though it may have some claims to be a good account of the relation between physical and mental disorder, cannot claim to meet the two criteria of valid extension. (shrink)