What Is Energy?

Extract

A Well-Known treatise on Physics written in 1892 begins with the words:

“A careful study of phenomena reveals to us only two things, or entities, as having an actual and objective existence. These two things are matter and energy. SO far as we can determine, therefore, these two entities, in their various forms, make up the whole of the physical universe.”

Such was the accepted doctrine in the latter part of the nineteenth century. I want to show how it originated and to what extent we have now departed from it.