A phenomenological theory of the vacancy jump is presented. It is shown on one hand that diffusion of crystalline structure defects should be described as evolution of statistical distributions. On the other hand, a strong conceptual relation is noted between the states of a diffusive particle and of a particle in a fluid. The crystal energy is then taken as a sum of interactions between pairs of atoms. Jaynes' generalization of Gibbs' statistical method is used. A self consistent field approximation gives a simple analytical expression for the jump rate, the migration energy and the migration volume of a vacancy. Numerical results agree clearly well with experimental data. This theory may be considered as a connection between fluid and solid mechanisms of diffusion.