The practical importance of machine learning of this latter kind has been underlined by the advent of knowledge-based expert systems. As their name suggests, these
systems are powered by knowledge that is represented explicitly rather than being implicit
in algorithms. The knowledge needed to drive the pioneering expert systems was
codified through protracted interaction between a domain specialist and a knowledge
engineer. While the typical rate of knowledge elucidation by this method is a few
rules per man day, an expert system for a complex task may require hundreds or even
thousands of such rules. It is obvious that the interview approach to knowledge acquisition
cannot keep pace with the burgeoning demand for expert systems; Feigenbaum
(1981) terms this the 'bottleneck' problem. This perception has stimulated the
investigation of machine learning methods as a means of explicating knowledge
(Michie, 1983).