- There are many mathematical expressions for energy, and many more expressions for what it can do. We know what all the particles are and can describe them as an entities by concrete terms such as mass and properties. But it does not seem that we can do that with energy.
Can energy be defined as an entity? The ability to do work is abstract, so is E=mc^2. It seems like energy is a catch-all phrase for something we can only describe in terms of other physical realities.