 Even independently of Einstein’s theory, however, one might see in hindsight that Newton’s application of Corollary VI, and his identification of what we’ve been calling quasi-inertial frames, already undermined the idea of an inertial frame. As we saw, Newton could treat a quasi-inertial system like that of Jupiter and its moons, not as essentially different from the system of the Earth and the Moon, but as a limiting case of such a system: as the whole system becomes sufficiently far from the central body, the differences among the accelerations of its parts toward their common center become negligibly small. In other words, Newton could treat such systems, and the mathematical description of the differences among them, as revealing the structure and workings of the sun’s gravity, rather than as questioning the fundamental distinction between uniform and accelerated motion. In that sense, he was not proposing the “complete physical equivalence” of uniformly accelerating systems with uniformly moving systems (inertial frames). But even Newton recognized, as we also saw, that the solar system as a whole might have an unknown and practically unknowable acceleration. In effect, he explained why his analysis of accelerations of the bodies in the system, among themselves, required no knowledge of any absolute accelerations. In the 19th century, Maxwell, without questioning the underlying framework of absolute space and time, pointed out that Corollary VI implied a kind of relativity of acceleration (1878, pp. 51–52). Viewing the foregoing in hindsight, more recent literature has suggested that the physics of the Principia did not really require the notion of inertial frame at all, and therefore that a weaker geometry than Newtonian spacetime would be a sufficient background structure for Newton’s dynamical reasoning (cf. Saunders, 2013). It is further suggested, following Cartan (1923, 1924), that the space-time structure and the gravitational field be unified in a curved space-time (cf. section 9, below), as a Newtonian version of general relativity (cf. Malament 2012, chapter 4; see also Knox, 2014 and Weatherall 2018). The relevant conceptual resources and mathematical techniques for such approaches, evidently, developed only in the aftermath of general relativity. We return to this theme in 2.5.