  To reach this conclusion Newton had to show that (1) the inertia of the fluid does indeed produce a resistance force proportional to its density, a force that (2) is independent of the tenacity (that is, surface friction) and the friction of the parts (that is, viscosity) of the fluid. Perhaps in part in emulation of the approach to centripetal forces that appeared to have succeeded so well in Books 1 and 3, the approach Newton takes in Book 2 is to develop, so far as he can, a generic mathematical theory of motion under resistance forces and then turn to experimental phenomena so that, in the words of Book 1, “it may be found out which conditions of forces apply” to different kinds of fluids. The theory in Book 1 is generic in that it examines centripetal forces that vary as different functions of the distance from the force center. The theory in Book 2 is generic in that it examines motion under resistance forces that vary as the velocity, the velocity squared, the sum of these two, and ultimately even the sum of two or three independent contributions, each of which is allowed to vary as any power of velocity whatever. Because Newton's goal was to reach a conclusion about the contribution to the total resistance made by the inertia of the fluid, and he recognized that surface friction and viscosity can contribute to the resistance as well, his empirical problem became one of disaggregating the inertial contribution from the total resistance, that is, the contribution that alone varies with the density of the fluid. Fortunately, because gravitational forces so totally dominate celestial motions, this need to disaggregate different sorts of forces did not arise in Book 3.