In defence of the Dirac theory of constraints

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.