Einstein: Energy and matter are interchangeable. If matter is energy, what then is energy?

If energy is a force interacting with another force to create the illusion of matter, then all matter is ultimately incorporeal. We are incorporeal interactions of energy.

Spinoza [writing in terms of 17th century understanding] also defined nature as one infinite and eternal substance, of which we are aware only of thought and extension; that is to say, energy and matter. Two attributes of one substance.