Many interesting machine learning problems with real world applications are hard.
With no additional restrictions learning to select an appropriate behavior or course of action to carry out in the real world is in the worst case computationally intractable. Furthermore, experience with real-world systems indicates that many tasks that seem to require intelligence fall at the difficult end of the spectrum. In explanation-based learning, the interaction among background knowledge axioms provides an ordering on the space of conceivable hypotheses. Plausible rather than logical inferencing is under study as defining relevant knowledge interactions. This points to a new semantics for declarative knowledge representations.