Methods or formulas can be applied for invention patents. For example, the synthesis method and formula of a chemical agent can be patented. However, purely theoretical things that do not produce effects or results cannot be patented. Otherwise, whatever others make using your theory in the future will infringe your copyright, which would be meaningless. For example, if we use "mechanical principles" (the theory you mentioned) and develop a certain device, you can't say that I infringe upon it. That's roughly what it means.