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The many-worlds interpretation has usually been regarded as immune to nonlocality, and similar claims have been made for the consistent-histories interpretation (1-3). However, for a thought experiment of the Hardy type, the argument for nonlocality in the usual interpretation can be extended to both these other interpretations.

It is shown that nonlocality gives rise to an undecidable proposition, meaning it cannot be proved true nor proved false from the usual assumptions, but is independent of them. A variation on the usual thought experiment is considered in which the observers are timelike separated, but the nonlocality fails to become a “precognition” effect because of this independence result.

An allegation of a flaw in the theory developed by Dirac for the Hamiltonian formation of singular dynamical systems is denied. It is shown that the Dirac theory is perfectly correct, and adequate as a Hamiltonian theory-but also there is a gain to be had by looking at the structural features involved in the original Lagrangian picture, including those 'canonical' features necessary for the purpose of quantisation; it is indicated how the controversy about first-class secondary constraints may be examined in this way.

A new approach is suggested to the teaching of analytical dynamics, in which the Lagrangian and Hamiltonian pictures are presented simultaneously. It is used to explain the connection between symmetries and conservation laws, and the passage to quantisation.

Some general considerations are applied to the Lenz vector, and a recent work on the subject is discussed.