Gopnik (1993) reports that young children do not understand their own beliefs before they understand the beliefs of other people. Gopnik argues that the young child develops a theory of mind which grows in complexity over time, and is simultaneously used to explain the child's own behaviour, and that of others, in terms of psychological states. Gopnik's position is called "theory theory". Harris (1989, 1991) proposes an alternative explanation of Gopnik's findings, in which the young child does not develop a theory of mind, but rather 'simulates' their intuitive understanding of their own psychological states in order to explain the behaviour of others. Harris' position is called "simulation theory". In the present study a third position is proposed, "first­ person theory theory", which occupies the middle ground between theory theory and simulation theory. First-person theory theory states that the young child begins to theorise about their own psychological states after having become reflectively aware of their own psychological experience or "stream of consciousness", and that the child's theory of mind is later used to explain the behaviour of others. The present study tested the developmental priority of self knowledge by comparing three-year-old children's understanding of false belief in others and representational change in themselves. The present study also tested the hypothesis that heightened cognitive involvement, caused by participation in the execution of the experimental task, would increase children's accuracy on questions about their own false beliefs to a greater degree than it would their accuracy on questions about another person's false beliefs. Subjects responded with greater accuracy to questions about where they had personally looked for a hidden object than they responded to all other test questions. The findings of the present study supported the developmental priority of self knowledge proposed by first-person theory theory and simulation theory, while constituting evidence against theory theory.