Without doing there is no sales. No one can help others sell a service or a product unless s/he has been in the trenches and has dealt with the disappointment of making a call, having a first meeting, making a presentation and then delivering the presentation to find out the business has been lost.
Without doing there is no marketing. The most ideal marketing plan is almost bound to fail if the team building the plan has never been part of successful team. Ideas that happen in a meeting room may be spectacular, but they don't have the backing of real life experience.
Without doing there is no personal effectiveness. I remember in the past reading book after book on personal effectiveness thinking this will be easy - my troubles are solved. Then the implementation happens and all the "what if" problems arise. It's after dealing with the "what if" problems that a real system can be developed.
Without doing there are no lean principles. In the lean philosophy, there is a concept called PDCA (plan, do, check, act) which is a system of trial and error with continual feedback and improvement. This is what Confucius was talking about. It takes real life to test any belief and then to improve on that belief.
All of these examples say to just get on with it - just start doing something. And then realize that forever the parts and pieces and systems will be changing and improving.