Gravitation can be casted as a physical theory by itself. The steps are: to define the interacting bodies, to define the interaction and its properties, to define the energies and forces involved, to show the self consistency of the mathematical structure involved, and a discussion of the (proportionality) relation between gravitational and inertial mass. The mathematical frame for this development is a 5-D quadratic space reference manifold: space, time and action (START). The principles, that is concepts defined through their use in the theory, are presented.