We prove that a gerbe with a connection can be defined on classical phase space, taking the U(1)–valued phase of Feynman path integrals as Čech 2–cocycles. A quantisation condition on the corresponding 3–form field strength is derived and proved to be equivalent to Heisenberg’s uncertainty principle.

As we know, observables are associated to Hermitian operators. Given one such operator A we can use it to measure some property of the physical system, as represented by a state Ψ. If the state is in an eigenstate of the operator A, we have no uncertainty in the value of the observable, which coincides with the eigenvalue corresponding to the eigenstate. We only have uncertainty in the value of...

Is there a plausible argument for external world skepticism? Robert Nozick’s well–known discussion focuses upon arguments which utilize the Sensitivity Requirement and the Closure Principle. Nozick claims, correctly, that no such argument succeeds. But he gets almost all the details wrong. The Sensitivity Requirement and the Closure Principle are compatible; the Sensitivity Requirement is incor...