 The will to power doctrine seems to claim that everything that exists rests fundamentally on an underlying basis of “power-centers”, whose activity and interactions are explained by a principle that they pursue the expansion of their power. But it is far from obvious what these “power-centers” are supposed to be, and much scholarly controversy concerns what kind of doctrine Nietzsche intended to advance in the first place. Some readers take it as Nietzsche’s version of a foundational metaphysics (see Heidegger 1961, Jaspers [1936] 1965, and for a sophisticated recent approach in the same broad vein, Richardson 1996; see also alternative accounts by Doyle 2019 and Remhof 2017). Others receive it as an anti-essentialist rejection of traditional metaphysical theorizing in which abstract and shifting power-centers replace stable entities (Nehamas 1985: 74–105, Poellner 1995: 137–98), or else as a psychological hypothesis (Kaufmann [1950] 1974, Soll 2015; Clark and Dudrick 2015), or a (quasi-)scientific conjecture (Schacht 1983; Abel 1984; Anderson 1994, 2012b). Opposing all such readings of the will to power as a doctrine in theoretical philosophy, Maudemarie Clark (2000, see also 1990: 205–44) reads the will to power as a strand of thought that makes no claim about the world, but instead expresses Nietzsche’s values. As we saw (3.B.i.), the idea that the expansion of power is good does have a better claim than other principles to systematize Nietzsche’s various value commitments, and different evaluative interpretations have been developed by Reginster (2006), Katsafanas (2013), Hussain (2011), and Richardson (2020: 53–80). But there are also a large number of other texts suggesting that Nietzsche’s main agenda was to argue that the psychological world—or the world as a whole—is fundamentally composed of centers of power exerting force against one another (see GS 13; BGE 23, 36, 259; GM II, 16–17; III, 13–15; as well as many passages from the notebooks). Nietzsche’s description of such “power centers” is sometimes fairly abstract, evoking mathematically characterized “force-centers” like those sometimes postulated in nineteenth century physics, but at other times, concrete psychological or biological entities (people, drives, organisms) are the things exerting will to power.