You can't be good at many things, mostly because people who spend all their time doing one thing will outdo you; so at best you'll be mediocre at many things. A

You can't be good at many things, mostly because people who spend all their time doing one thing will outdo you; so at best you'll be mediocre at many things. At worst, you'll think you're good at all those things. Be mediocre at many things and specialize to some extent on few things.