In thermodynamics, Herbert B. Callen (1919-1993) was an American physicist, of the MIT school of thermodynamics, noted for the 1985 second edition of his thermodynamics textbook Thermodynamics an Introduction to Thermostatistics, one of the most-referenced thermodynamics publications, especially in the physics community.

“The development of the principle of conservation of energy has been one of the most significant achievements in the evolution of physics. The present form of the principle was not discovered in one magnificent stroke of insight but has been slowly and laboriously developed over two and a half centuries. The first recognition of a conservation principle, by Leibnitz in 1693, referred only to the sum of the kinetic energy (½mv²) and the potential energy (mgh) of a simple mechanical mass point in the terrestrial gravitational field. As additional types of systems were considered, the established form of the conservation principle repeatedly failed, but in each case it was found possible to revive it by the addition of a new mathematical term - a "new kind of energy." Thus, consideration of charged systems necessitated the addition of the Coulomb interaction energy (Q1Q2/r) and eventually of the energy of the electromagnetic field. In 1905, Einstein extended the principle to the relativistic region, adding such terms as the relativistic rest-mass energy. In the 1930's Enrico Fermi postulated the existence of a new particle, called the neutrino, solely for the purpose of retaining the energy conservation principle in nuclear reactions. Contemporary research in nuclear physics seeks the form of interaction between nucleons within a nucleus in order that the conservation principle may be formulated explicitly at the subnuclear level. Despite the fact that unsolved problems of this type remain, the energy conservation principle is now accepted as one of the most fundamental, general, and significant principles of physical theory.”

— Herbert Callen (1960), Thermodynamics (Ѻ)