"Four causes" refers to an influential principle in Aristotelian thought whereby explanations of change or movement are classified into four fundamental types of answer to the question "why?" Aristotle wrote that "we do not have knowledge of a thing until we have grasped its why, that is to say, its explanation." While there are cases where classifying an explanation is difficult, or in which classes of explanation might merge, Aristotle was convinced that his four classes of explanation provided an analytical scheme of general applicability.Aitia, from Greek αἰτία was the word that Aristotle used to refer to the concept of explanation.