"In the system to which the quantum mechanical formalism is applied,
it is of course possible to include any intermediate auxiliary agency
employed in the measuring processes. . . . The only significant point is
that in each case some ultimate measuring instruments, like the scales
and clocks which determine the frame of space-time coordination—
on which, in the last resort, even the definition of momentum and
energy quantities rest—must always be described entirely on classical
lines, and consequently be kept outside the system subject to quantum