Modern physicists are agreed that visible motion, heat, electricity, magnetism, and radiance (radiant light and heat) are forms of actual _energy_ which are correlated and capable of mutual conversion. Any one form may, by suitable processes, be transformed, wholly or in part, to an equivalent amount of any other form of energy. So much is generally accepted by scientific men. But in regard to the primary _force_ or forces in which these forms of energy have their origin, there is not the same agreement among physicists. Some regard gravitation, cohesion, and chemical affinity as the three primary forces of nature; while others suggest that the last two are related with and probably derived from the first. There is also a respectable school of physicists who teach that atomic attractions and repulsions are the universal cosmic forces which originate all molecular and mechanical motions. Then, again, each of these forms of force have their special advocates. On the one side it is affirmed, as an important generalization, that all primary force is _attractive_; "there is no such thing in nature as a primary repulsive force."[268] Universal attraction is the one world-forming and world-conserving energy. On the other side it is contended that gravitation is not a primary, but a secondary and derivative force, and that the grand primal force is a universal force of _repulsion_.[269] It is beyond our province to discuss the merits of these conflicting theories. Our position is that no purely physical hypothesis is adequate to account for the conservation of the universe, and therefore it is of little consequence to our argument which of the above theories may find most favor with scientific men. The tendency of modern scientific thought is toward the conception of "one primordial form of matter, and but one primary form of force," as the simplest basis upon which a physical theory of inanimate nature can be erected. The ultimate nature of this one primary force is a question for pure metaphysics. From the stand-point of physical science it can only be thought "as a _pull_ or a _push_ in a straight line."[270] Universal attraction or universal repulsion must be the ultimate dynamical conception for the pure physicist.