The lexical development of four infants was recorded by their parents in diaries. In a selective imitation situation, individualized for each child, the responses of the children were compared with semantic predictions made on the basis of one of 14 hypothesized rules, and with the semantic alternatives available from the child's lexicon. It was found that, in general, it is possible to predict what a child will say and when on the basis of the structure of a referential event. The research is deemed supportive of the notion that the distinction between information and certainty is the psychological basis for the distinction between assertion and presupposition in language, i.e., changing, informative elements are expressed on a background of unexpressed certainties. (JB)