  Postulating the identification of $\psi^*(x, t) \psi(x,t)$ with a physical
probability density is unsatisfactory conceptually and overly limited
practically. For electrons, there is a simple, calculable relativistic
correction proportional to $\nabla \psi^* \cdot \nabla \psi$. In particular,
zeroes of the wave function do not indicate vanishing probability density of
presence. Effects of this kind arise generically in Lagrangian-based theories
implementing the particle concept.
