I am thinking in the mechanical context.
Everywhere I research (e.g. Wikipedia) the law of conservation of energy is stated only for the special case of an isolated system. I am wondering if conservation of energy holds beyond that special case; it seems that it should. After all, if a property holds only under special cases then the property is not a law.
Reading Feynman's lecture 14 volume I, I understand that if only conservative forces act on an object its total energy remains unchanged. For example, a falling object subject only to gravity has a constant sum of kinetic and potential energies. However, the system consisting of just this object is not an isolated system because it is subject to the external force of gravity. It seems this is an instance of conservation of energy holding outside the special case of an isolated system.
Feynman argues that at the fundamental level all forces are in fact conservative. This implies that at the fundamental level conservation of energy applies to all systems. Is this true? If so, why is conservation of energy continually stated under the special case of an isolated system?
(this site's "energy-conservation" tag says "the amount of energy in a system is constant" implying the system need not be isolated, further confusing me)