The four causes are, in Aristotelian thought, categories of questions that explain "the why's" of something that exists or changes in nature. The four causes are the: material cause, the formal cause, the efficient cause, and the final cause. Aristotle wrote that "we do not have knowledge of a thing until we have grasped its why, that is to say, its cause." While there are cases in which classifying a "cause" is difficult, or in which "causes" might merge, Aristotle held that his four "causes" provided an analytical scheme of general applicability.