 Questions about causation form yet a fourth important category of issues in the “new” metaphysics. Of course, discussion of causes go back to Ancient Philosophy, featuring prominently in Aristotle's Metaphysics and Physics. But Aristotle understood ‘cause’ in a much broader sense than we do today. In Aristotle's sense, a ‘cause’ or ‘aiton’ is an explanatory condition of an object—an answer to a “why” question about the object. Aristotle classifies four such explanatory conditions—an object's form, matter, efficient cause, and teleology. An object's efficient cause is the cause which explains change or motion in an object. With the rise of modern physics in the seventeenth century, interest in efficient causal relations became acute, and it remains so today. And when contemporary philosophers discuss problems of causation, they typically mean this sense.