The work offers a concise, elementary exposition of energy as a physical quantity, opening with atoms and a definition of energy and explaining how mechanical energy converts into heat. It surveys the various forces of nature, formulates the law of conservation, and catalogues the ways energy transforms from one form to another. A historical sketch traces development of the doctrine and introduces the concept of dissipation, while a final chapter considers the role of living systems within an energy-governed universe. Appended essays extend the principle to biological and psychological domains by discussing correlations between physical, chemical, vital, nervous, and mental forces.