To know how good you are at something requires the same skills as it does to be good at that thing. Which means if you are absolutely hopeless at something

To know how good you are at something requires the same skills as it does to be good at that thing. Which means if you are absolutely hopeless at something you lack exactly the skills that you need to know that you are absolutely hopeless at it. And this is a profound discovery. That most people who have absolutely no idea what they're doing have no idea that they have no idea what they're doing. It explains a great deal of life. It explains particularly Hollywood. - John Cleese