The heterogeneity of clinical syndromes subsumed by diagnostic criteria for major depressive disorder (MDD) is regarded by some as a reason to abandon or modify the criteria. However, heterogeneity may be unavoidable because of the biopsychosocial complexity of depression. MDD may be characterised by complexities that cannot be distilled down to any brief set of diagnostic criteria. Psychiatrists and psychiatric epidemiologists may need to revise their expectations of this diagnosis in order to avoid over-estimating its ability to guide the selection of treatments and prediction of prognosis. An opposing perspective is that of reification, in which the diagnosis is viewed as being more real than it really is. The concept of rheostasis may help to explain some features of this condition, such as why major depressive episodes sometimes seem understandable or even adaptive (e.g. in the context of bereavement) whereas at other times such episodes are inexplicable and maladaptive.