All matter contains energy. It contains energr in respect of atomic and molecular motion, of interatomic forces, of magnetic forces, of magnetic and electric fields in the nucleus as well as outside it, of its physical state, and in many other ways.In order to calculate the total energy contained in a sample we would first have to define a reference zero point. Such a zero would in the end be quite arbitrary. It is sufficient for us to consider only changes in the energy of a system, which, as we have seen, may be brought about principally by thermal or mechanical means. This and previous ideas are summarized in the formal statement of the First Law