Since the early 1980s there has been a surge of interest in the cognitive basis of our common sense ‘theory of mind’ and how it develops. According to our common sense, other people act because they have mental states of various kinds, for example, intentions, desires, beliefs, hopes, etc.; furthermore, such states have contents, for example, the belief that it is raining has the content ‘it is raining’ and the desire to avoid paying taxes has the content ‘avoid paying taxes.’ Contents individuate mental states (for example, determine which belief or which desire a person has) and play a critical role in causing behavior. Even preschool children have been found to attribute these sorts of mental states to other people and to treat these states-with-contents as causes of behavior. Children who are learning disabled can also employ such a theory of mind, suggesting that it is not the result of a general intellectual ability. Children with autistic disorder are selectively impaired in theory of mind ability even though their general intellectual abilities may be normal. Theory of mind is a key cognitive ability comprising the human social instinct. The psychological mechanisms underlying this ability are of intense current interest but their nature remains controversial.