The search for scientific knowledge extends far back into antiquity. At some point in the past, at least by the time of Aristotle, philosophers recognized that a fundamental distinction should be drawn between two kinds of scientific knowledge — roughly, knowledge ''that'' and knowledge ''why''. It is one thing to know ''that'' each planet periodically reverses the direction of its motion with respect to the background of fixed stars; it is quite a different matter to know ''why''. Knowledge of the former type is descriptive; knowledge of the latter type is explanatory. It is explanatory knowledge that provides scientific understanding of the world. (Salmon, 1990)