Theoretical patent

Any method or formula can apply for a patent for invention. For example, the synthetic method and formula of a chemical agent can be patented. However, a purely theoretical thing can't be patented without any effect and achievement. Otherwise, everything others make with your theory is infringing, and it will be boring. For example, we used the "mechanical principle" (the theory you mentioned) and then developed some kind of device. You can't call me an infringement. That's about it.