account, and which should be included in any attempt to unify knowledge. These, commonly known as the conservation of energy and the indestructibility of matter, are undoubtedly, so far as we can be certain of anything, of universal validity. Others again, such as the law of gravitation, while possibly not of such great theoretical certainty, are expressions of fact to which no exception has ever been discovered. The former two great principles are the starting point of Spencer's unification, yet, as usually expressed, they are not in the form that possesses the greatest philosophical value. The conservation of energy in particular is often regarded as an empirical law, which the last generation of physicists usually expressed by saying that the sum-total of kinetic and potential energy is constant. While, for the solution of certain problems of mechanics, the conception of potential energy possesses methodological value, it is out of harmony with ideas suggested from a wider point of view. From the standpoint of philosophy, potential energy is a meaningless figment, to be ranked with electric fluids, tubes of force, and other devices useful as a short cut to mathematical calculation.
To Spencer belongs the credit of first formulating a view now obtaining considerable recognition among physicists, that all energy is actual. Just as the energy of heat is not regarded as lost, but as actually existent in the motion of molecules, so gravitational energy, commonly called potential, must be regarded as actually existent in ether stresses and strains. The conception of the ether as a receptacle of energy is implicit in any theory of radiation, and generally in modern electro-magnetic theory, so present-day physicists have no logical reason for declining to accept this valuable addition to the theory of their subject which Spencer supplied so many years ago.
The philosophical form of the idea supplies one reason why Spencer adhered to his original term, the persistence of force, instead of the modern equivalent, the conservation of energy. Energy is a complex, and, as usually formulated, the latter principle requires the division into potential and kinetic to give methodological completeness. Spencer's principle, the persistence of force, includes this and much more. For physical cal-
