Using an interdisciplinary approach, including historical semantics, medicine, natural philosophy and law, the book considers a neglected field of social and medical history and makes an original contribution to the problem of a shifting concept such as 'idiocy'. The book considers the semantics of intellectual disability (ID) by looking at the words and labels used across time and place for conditions that might be subsumed by the umbrella-term 'intellectual disability' in modern Western society. The book discusses concepts of ID in medieval natural science, that is, anatomical and medical texts, now termed as the neurological foundations. Turning from the material aspects of neurology to the immateriality of psychology, it treats mind and soul in relation to ID. Discussing the theme of childishness, the book considers the legal position of persons with ID. The question of whether a legal case related to mental illness or ID is analysed. Thinking about legal agency returns to the themes of idiocy and infancy. The book then looks at the socio-cultural implications of ID through the lens of court fools, pets and entertainers. An overview of the link between court fools, idiots and social theories of dominance leads on to classical antiquity and the origin of 'fools', with the fully fledged medieval court fools noticeable and remarkable for 'foolish' behaviour rather than medicalised traits.