Needham discusses Yin and Yang together with Five Elements as part of the School of Naturalists. He says that it would be proper to begin with Yin and Yang before Five Elements because the former: "lay, as it were, at a deeper level in Nature, and were the most ultimate principles of which the ancient Chinese could conceive. But it so happens that we know a good deal more about the historical origin of the Five-Element theory than about that of the Yin and the Yang, and it will therefore be more convenient to deal with it first. " He then discusses Zou Yan (鄒衍; 305 – 240 BC) who is most associated with these theories. Although Yin and Yang are not mentioned in any of the surviving documents of Zou Yan, his school was known as the Yin Yang Jia (Yin and Yang School) Needham concludes "There can be very little doubt that the philosophical use of the terms began about the beginning of the -4th century, and that the passages in older texts which mention this use are interpolations made later than that time. "