Nearly half a century after this assessment by Einstein of the state of physical theory we still have today a dualistic theory with two kinds of physical realities: matter on the one hand, and electromagnetic field and gravitation on the other hand. Although in modern quantum field theory both entities are described mathematically by fields, they are distinct fields, a field A _µ (x) for the electromagnetic field, for example, and fields Ѱ_i (x) for the electrons, protons, etc.—or a curved space and fields Ѱ_i (x), to describe the interaction of matter and gravitational field. The action (of the world) in the first case is a sum of three terms, one for the matter field (or fields), one for the radiation field, and a local coupling or interaction term involving the products of both kinds of fields.