Theory of Mind (ToM) is the cognitive achievement that enables us to report our propositional attitudes, to attribute such attitudes to others, and to use such postulated or observed mental states in the prediction and explanation of behavior. Most normally developing children acquire ToM between the ages of 3 and 5 years, but serious delays beyond this chronological and mental age have been observed in children with autism, as well as in those with severe sensory impairments. We examine data from (...) studies of ToM in normally developing children and those with deafness, blindness, autism and Williams syndrome, as well as data from lower primates, in a search for answers to key theoretical questions concerning the origins, nature and representation of knowledge about the mind. In answer to these, we offer a framework according to which ToM is jointly dependent upon language and social experience, and is produced by a conjunction of language acquisition with children's growing social understanding, acquired through conversation and interaction with others. We argue that adequate language and adequate social skills are jointly causally sufficient, and individually causally necessary, for producing ToM. Thus our account supports a social developmental theory of the genesis of human cognition, inspired by the work of Sellars and Vygotsky. How are we to decide whether to take reason to be an essentially private thing that can, however, turn on a public display when it chooses to do so, or, like conversing, to be an essentially social skill, which can, however, be retained a while through periods of solitary confinement? (Annette Baier, The Commons of the Mind, p. 5). The squirrel does not infer by induction that it is going to need stores next winter as well (Wittgenstein, On Certainty§ 287). The problem of thought and language thus extends beyond the limits of natural science and becomes the focal problem of historical human psychology, i.e. of social psychology. Consequently, it must be posed in a different way (Vygotsky, Thought and Language, p. 51). From the very first days of the child's development his activities acquire a meaning of their own in a system of social behaviour and, being directed towards a definite purpose, are refracted through the prism of the child's environment. The path from object to child and from child to object passes through another person. This complex human structure is the product of a developmental process deeply rooted in the links between individual and social history (Vygotsky, Mind in Society, p. 30). (shrink)