 Even within physical science, reduction between different levels of explanation is problematic—indeed, it is almost always so. Chemistry is supposed to have been reduced to quantum mechanics, yet people still argue over the basic question of how quantum mechanics can describe the shape of a molecule. The statistical mechanics of a fluid reduces to its thermodynamics in the limit of infinitely many particles, yet that limit breaks down near the critical point, where liquid and vapour merge, and where we never see a continuum no matter how distantly we observe the particles … . The geometrical (Newtonian) optics of rays should be the limit of wave optics as the wavelength becomes negligibly small, yet … the reduction (mathematically similar to that of classical to quantum mechanics) is obstructed by singularities … .