 Positive Evidence as Negative Evidence. Relatedly, learners  can also exploit positive evidence as to which strings occur in the  pld as a source of negative evidence — again in a  tentative and revisable   way.[15]  Suppose that the child's grammar generated two strings as appropriate in a given kind of context, but that only one sort of string was ever produced by those around her. The fact that only strings of the first kind occur is in this case negative evidence — defeasible, to  be sure, but negative evidence nonetheless.