  Furthermore, it is worthwhile to distinguish between levels of intentionality. Many of an individual’s psychological states with intentionality (e.g., beliefs) are about (or represent) non-mental things, properties and states of affairs. Many are also about another’s psychological states (e.g., another’s beliefs). Beliefs about others’ beliefs display what is known as ‘higher-order intentionality.’ Since the seminal (1978) paper by primatologists David Premack and Guy Woodruff entitled “Does the chimpanzee have a theory of mind?”, under the heading ‘theory of mind,’ much empirical research of the past thirty years has been devoted to the psychological questions whether non-human primates can ascribe psychological states with intentionality to others and how human children develop their capacity to ascribe to others psychological states with intentionality (cf. the comments by philosophers Jonathan Bennett, Daniel Dennett and Gilbert Harman to Premack and Woodruff’s paper and the SEP entry  folk psychology: as a theory).