To understand modern science we cannot talk merely about theories. We must recognize the indispensable role of models. By theories we mean basic theories in physics, as described in scientific textbooks. Each basic theory describes a separate aspect of physical reality: movement under forces, gravity, electricity and magnetism, heat phenomena, and the like. Interactions described by basic theories are universal, that is, they apply to every element of physical reality. Every two elements of mass attract each other with the force described by the theory of gravity. However small is that force, however small or remote are the masses, the theory assigns that force some value, even if it is non-measurably small. Empirical verification of statements about individual interactions between concrete objects is difficult not only because the effects caused by a particular interaction may be non-measurable, but also because we cannot measure a selected interaction, but rather a resultant of many interactions, some of them we may not even know.