It is shown that it is possible to give a discrete formulation to the laws of physics, starting directly from experimental laws, i.e. avoiding whatever discretization process of differential equations. This is made possible by the consideration of global variables as the primitive form of all physical variables and not as integrals of field variables on lines, surfaces and volumes. This inversion of role, i.e. global variables as generators of field variables, leads to a general classification diagram of physical variables of every (classical) physical theory and to the introduction of a numerical method, called the "cell method". (More information at http://discretephysics.dic.units.it) )