The paper examines the Dirac-Pauli differences concerning the order of the primary differential equation of an elementary massive quantum particle. The analysis relies on several self-evident constraints that an acceptable quantum theory must satisfy, like conservation laws, compatibility with Maxwellian electrodynamics and the correspondence principle. The dimension of the quantum function that is used in the Lagrangian density of a given quantum theory together with the corresponding differential equations play an important role in the reasoning procedure. The paper proves that Dirac was right and that second-order quantum theories like the Klein-Gordon equation and the electroweak theory of the W± bosons do not satisfy fundamental constraints. This outcome is inconsistent with the Standard Model of Particle Physics.