 Harman’s conclusion that new knowledge can undermine old knowledge can be applied to the surprise test paradox: The students lose knowledge of the test announcement even though they do not forget the announcement or do anything else incompatible with their credentials as ideal reasoners. A student on Thursday is better informed about the outcomes of test days than he was on Sunday. He knows the test was not on Monday and not on Wednesday. But he can only predict that the test is on Friday if he continues to know the announcement. The extra knowledge of the testless days undermines knowledge of the announcement.