"It will be objected here, perhaps, that a perpetual motion proved impossible for _purely mechanical actions_, is perhaps not so when the influence of _heat_ or of electricity is employed. But can phenomena of heat or electricity be thought of as due to anything else than to _certain motions of bodies_, and as such must they not be subject to the general laws of mechanics?"[54] These examples, which might be multiplied by quotations from recent literature indefinitely, show that a tendency to explain all things mechanically actually exists. This tendency is also intelligible. Mechanical events as simple motions in space and time best admit of observation and pursuit by the help of our highly organised senses. We reproduce mechanical processes almost without effort in our imagination. Pressure as a circumstance that produces motion is very familiar to us from daily experience. All changes which the individual personally produces in his environment, or humanity brings about by means of the arts in the world, are effected through the instrumentality of _motions_. Almost of necessity, therefore, motion appears to us as the most important physical factor. Moreover, mechanical properties may be discovered in all physical events. The sounding bell trembles, the heated body expands, the electrified body attracts other bodies. Why, therefore, should we not attempt to grasp all events under their mechanical aspect, since that is so easily apprehended and most accessible to observation and measurement? In fact, no objection _is_ to be made to the attempt to elucidate the properties of physical events by mechanical _analogies_. But modern physics has proceeded _very far_ in this direction. The point of view which Wundt represents in his excellent treatise _On the Physical Axioms_ is probably shared by the majority of physicists. The axioms of physics which Wundt sets up are as follows: 1. All natural causes are motional causes. 2. Every motional cause lies outside the object moved. 3. All motional causes act in the direction of the straight line of junction, and so forth. 4. The effect of every cause persists. 5. Every effect involves an equal countereffect. 6. Every effect is equivalent to its cause. These principles might be studied properly enough as fundamental principles of mechanics. But when they are set up as axioms of physics, their enunciation is simply tantamount to a negation of all events except motion.