In expert systems
or knowledge bases
having an inference engine
, backward inference is inference
that occurs at the time a question
is posed to the system. For example, if a system had a rule stating that every raven
is black, and it knew that Joe is a raven, but it waited until it was asked "What color is Joe?" before concluding that he is black in color, that would be backward inference. A terse way of explaining backward inference is that it is inference that happens at query
Contrast backward inference with forward inference, which is inference that happens at assert time.