 Theory-theory has its roots in a paper by Premack and Woodruff (1978), which argued that certain problem-solving behavior observed in chimpanzees should be taken as evidence that they possess a theory of mind, as evidence that they are able to make predictions about the behavior of others that impute to them unobservable mental states. Premack and Woodruff take this imputation to be a rather primitive and unsophisticated reaction to the observation of certain behavior, so natural in both humans and chimpanzees that it would take an effort to suppress  it.[14]