  An explanation for a state of affairs must specify some feature or some object (in general, some abstract or concrete entity) which is responsible for it. The entity responsible is, Aristotle submits, a cause (aitia or aition, words used interchangeably by  Aristotle).[4]  Different explanations of a single state of affairs are possible, and indeed usually necessary, because there are different ways of being responsible for distinct facets of the same state of affairs. The varieties of responsibilities are grouped by Aristotle under four headings, the so-called four causes.