  The four (electro-magnetic, weak, strong and gravitational) interactions are
described by singular Lagrangians and by Dirac-Bergmann theory of Hamiltonian
constraints. As a consequence a subset of the original configuration variables
are {\it gauge variables}, not determined by the equations of motion. Only at
the Hamiltonian level it is possible to separate the gauge variables from the
deterministic physical degrees of freedom, the {\it Dirac observables}, and to
formulate a well posed Cauchy problem for them both in special and general
relativity. Then the requirement of {\it causality} dictates the choice of {\it
retarded} solutions at the classical level. However both the problems of the
classical theory of the electron, leading to the choice of ${1\over 2}
(retarded + advanced)$ solutions, and the regularization of quantum field
teory, leading to the Feynman propagator, introduce {\it anticipatory} aspects.
The determination of the relativistic Darwin potential as a semi-classical
approximation to the Lienard-Wiechert solution for particles with
Grassmann-valued electric charges, regularizing the Coulomb self-energies,
shows that these anticipatory effects live beyond the semi-classical
approximation (tree level) under the form of radiative corrections, at least
for the electro-magnetic interaction.
