"Zhu" Xi maintained that all things are brought into being by the union of two universal aspects of reality: "qi" (气 Wade-Giles: ch‘i), sometimes translated as vital (or physical, material) force; and "li" (理), sometimes translated as rational principle (or law). The source and sum of "li" is the "Taiji" (太极 Wade-Giles: T‘ai Chi), meaning the Supreme Ultimate. The source of "qi" is not so clearly stated by Zhu Xi, leading some authorities to maintain that he was a metaphysical monist and others to maintain that he was a metaphysical dualist.