Thales may or may not have considered the cosmos as “matter” _and_ “soul” or “moving power”. In any event the pre-Socratic Ionian philosophers recognized no distinction between matter and soul in our modern sense. The moving power of nature (soul) was as much a material substance as gross matter itself, only more rarefied, more elusive. It was equivalent to the “energy”—electricity—of modern science. Here we have, then, the answer to the question: “What is electricity?” It is _energy_—“the only real existing entity, the primary conception, which exists for us because our senses respond to it.” “All sense perceptions are exclusively energy effects.” This is the answer to the question: “What are the Hertzian waves, used in ‘wireless’?” It is the answer also to the question: “What is light?” as well as “What is electricity?” By carrying the explanation of the beam of light and the electromagnetic wave (like that of the radio communication station or that surrounding a power transmission line) back to the _energy_ field (or, less accurately, the field of force), we have carried it back, as Dr. Steinmetz well declared, as far as possible, “to the fundamental or primary conceptions of the human mind, the perceptions of the senses.” All that we know of the world is derived from the _perceptions of our senses_, which are for us the only _real facts_, all things else being conclusions from them; and “all sense perceptions are exclusively _energy_ effects.” Electricity is an energy effect, perceived by our senses. No other definition or explanation can or need be given, since _energy is the primary conception_. And this explains also what matter is, since _energy_ and _matter_ are interchangeable—or equivalent—terms. What we call electricity is one of the _effects of energy_ on our senses. In itself, it _is_ energy, the stuff that matter is made of; at once the “moving power” and the thing moved. Everything has been said that can be said now as to what electricity _is_: our concern in the remainder of this volume will be to discover what electricity _does_ and how it acts. * * * * * The reader of this little book who may be more or less familiar with larger volumes dealing with electricity, energy, electrons, electromagnetic waves or oscillations, magnetic and dielectric fields (usually combined), light-waves, etc., will notice that no mention has been made of the classical ether hypothesis, the universal _plenum_ in which energy is said to be stored, and in which transverse waves of light are said to occur, ether atoms or vibrations moving at right angles (perpendicularly) to the light-beam.