The document discusses the basic principles and theories of hydrotherapy. It explains that hydrotherapy is based on the ancient Indian philosophy of panchamahabhuta theory which states that everything is composed of five elements - ether, air, fire, water, and earth. It also discusses the theory of action and reaction in hydrotherapy, where the application of water to the skin causes an immediate effect, followed by a secondary reaction and eventual remote effect on the body's processes. Diagrams are included to illustrate the vital movements that occur from cold and warm hydrotherapy applications.