 This solution implies there can be disagreement amongst ideal reasoners who agree on the same impersonal data. Consider the colleagues of the teachers. They are not amongst those that the teacher targets for surprise. Since ‘surprise’ here means ‘surprise to the students’, the teacher’s colleagues can consistently infer that the test will be on the last day from the premise that it has not been given on any previous day. But these colleagues are useless to the students as informants.