In order to speed up, and simplify, automated learning of rules by a neural network making use of fuzzy logic, data (120) from a system is analyzed by a teaching data creation means (140). This groups the data into clusters and then selects a representative data item from each group for subsequent analysis. The selected data items are passed to a rule extraction means (180). This investigates relationships between the data items, to derive rules, but eliminates rules which have only an insignificant effect on the system. The result are candidate rules which are stored in a first rule base (200). The candidate rules are then compared with rules in a second rule base (240) to check for duplication and/or contradiction. Only those rules which are not duplicated and not contradictory are stored in the second rule base (240). Hence, when fuzzy inference is used to control the system on the basis of rules in the second rule base (240), only valid rules which provide a significant effect on the system are used.