The biomedical model posits that all physical and mental illnesses are due to measurable, physiological deviations from normal, healthy functioning. The notion that all mental illnesses are diseases of the brain, first advanced by Wilhelm Greisinger, is a strict application of the biomedical model within a psychological context. The model's philosophical and historical origins, including the scientific principle of reductionism, are discussed. The biomedical model's exclusion of psychological and social factors as essential to mental illness has led to a number of criticisms of the model. Alternatives to the biomedical model in psychological science include Engel's biopsychosocial model, which provides for inclusion of psychological and social factors in definitions of disease. The biomedical model has had a profound influence on medical and psychological science and continues to drive research efforts directed at identifying physiological underpinnings of psychological disorders.