The development of mental terms: pragmatics or semantics?

The distinctions between the mental terms, know, think and sure was examined in an experiment with 60 children between three and six years of age. The children were required to find an object hidden in one of two places. Their only clues were two statements involving contrasting mental terms, with each statement referring to one of the possible hiding places. Results showed a significant improvement with age for the know-think and sure-think contrasts, with think treated as a less reliable index of location than both know and sure by four to five years of age. No change with age was found for know-sure contrast. It is concluded that by four to five years of age, children recognize the function of mental terms to express degrees of certainty, and that this understanding is probably not based on the factive properties of the terms.