Merton Miller put it this way: "The M&M [Modigliani and Miller] model was much harder to implement empirically [than the Capital Asset Pricing Model].... It's very difficult to prove `empirically' the M&M propositions holding even as approximations. You ... can't hold everything else constant." (Miller interview) In other words, you can't empirically identify in an unequivocal way the "risk classes" described in appendix A.

Donald MacKenzie, An Engine, Not a Camera, loc. 389