Whenever we need to come up with ways to affect the system, we should go back to the mechanism by which system operates. A mechanism, by definition, is:

  1. input variables
  2. equations/laws by which the inputs are transformed
  3. output variables

If we know 1., we can immediately tell which factors affect the system - the ones which are used as input values! If we know 2. we can tell how those factors affect the system.