 Undoubtedly the newest thing about Neo-Confucianism was its metaphysics: while Confucius and Mencius had apparently assumed the reality of the world, they had not felt obliged to explain that assumption theoretically, even in passing. In the wake of Buddhism’s sway during much of the Tang dynasty, Neo-Confucians of the Song and later dynasties explicitly accounted for the reality of the world by positing a generative substantial force, qi (: ki), capable of assuming a variety of forms: liquid, solid, and ethereal. This generative force was the Neo-Confucian response to Buddhist claims regarding the essential insubstantiality of the world. Providing a sort of intelligible order to the world of generative force was the Neo-Confucian conception of an essential rational principle (C: li; J: ri) inhering in all things. Together, rational principle and generative force constituted the basic ingredients of a variety of expressions of the Neo-Confucian affirmation of the reality of the world. Theorists often differed regarding the priority of one notion in relation to the other, or whether there was in fact any priority between them at all, but rarely was it the case that later Confucian forays into metaphysical speculation abandoned either of the two metaphysical ingredients entirely.