Aristotle, "Book 5, section 1013a", Metaphysics, Translated by Hugh Tredennick Aristotle in 23 Volumes, Vols. 17, 18, Cambridge, MA, Harvard University Press; London, William Heinemann Ltd. 1933, 1989; (hosted at perseus.tufts.edu.) Aristotle also discusses the four "causes" in his Physics, Book B, chapter 3.
