1. A method of inferring a suitable answer to an interrogation utilizing a plurality of knowledge units in accordance with corresponding attributive information, which comprises the steps of: (a) accessing the knowledge units and corresponding attributive information from knowledge accumulation means, said corresponding attributive information including frequency-of-use information; (b) rearranging the accessed knowledge units and corresponding attributive information into a frequency-of-use order based on the frequency-of-use information; (c) storing the rearranged knowledge units and corresponding attributive information in work domains of a memory; (d) searching the knowledge units stored in step (c) in accordance with the frequency-of-use information successively from knowledge units having a greater frequency-of-use to knowledge units having a lesser frequency-of-use; (e) selecting the suitable answer based on one of (1) knowledge units stored in the work domains, and (2) knowledge units generated by inference of syllogism; (f) updating the frequency-of-use information of the selected knowledge units.