As I have studied Mechanics and Thermodynamics, it appears that many of the concepts and ideas behind Physics depend on the Law of Conservation of Energy. The textbooks, however, don't explain where does this Law come from and why it is treated like absolute truth. Conservation of Energy is always defined in terms of itself, in other words, many textbooks justify C.E. by using C.E. or simply implying it is true. However, I am sure that there is a justification that I have yet to see. Noether's theorem appears to be a solution (However, I do not have any knowledge about it.) but Conservation of Energy was already a thing before her theorem. How did physicists come up with this idea and why did they accept it as general truth at some point?