An expert system, in particular for medical diagnosis, includes a memory 2 for storing a plurality of hypotheses. The hypotheses are arranged in a first and second disjoint group of hypotheses. The memory stores questions for rejecting hypotheses of the second group. An output 3 is used for supplying questions to a user. A input 4 is used for receiving initial data and answers to questions. A processor 5 is programmed to select questions from the stored questions for those hypotheses from the second group that are possible in dependence on the initial data. The processor also determines from answer(s) received in response to outputting the selected questions whether at least one of the hypotheses of the second group is possible. In response to determining that no hypothesis of the second group is possible, the processor supplies a most likely hypothesis of the first group.