 There are several theoretical approaches to delusion formation which attempt to explain the surface features of delusions by reference to unusual experiences, reasoning biases, neuropsychological deficits, motivational factors, and prediction errors, but the task of describing the behavioral manifestations of delusions, and reconstructing their etiology is made difficult by the variation observed both in the form and in the content of delusions. When the distinction between functional and organic delusion was still widely accepted, functional delusions were primarily explained on the basis of psychodynamic factors, whereas organic delusions mostly received a neurobiological explanation. At the present stage of empirical investigation in the formation of delusional states, the received view is that all delusions are due to neuropsychological deficits, which might include motivational factors.