"Our only limitations are those we set up in our own minds." -- Napoleon Hill
Truer words were never spoken, or in this case written. These words were taken right out of Napoleon Hill's much-quoted book Think and Grow Rich.
How many times have you seen or been a participant of this scenario? Person A says something like, "I have this great opportunity for you!" "Oh yeah! What is it?" is the gist of what Person B says. Then after Person A has described the nature of the opportunity, person B says something like "but that's impossible!" or "that would never work!" or something to that effect.
Then Person A goes ahead with the opportunity, whatever it is, and becomes hugely successful, because he know it was possible and that he could do it. Person B believed it was impossible and so had nothing to do with it, he limited himself to what he already had by refusing to see what he could have with a little vision and courage.
Now Person A is hugely successful and doing a job he loves, while Person B is still in that same dead-end go-nowhere job he hates and struggling to pay the bills each month.
What's the difference? Person A knew that if he believed in himself and worked at it, he could achieve something better. Person B believed getting something better was impossible and too risky, so he closed his mind to the possibilities and opportunities all around him. Person A, however, could see them and take advantage of enough of them to have a good, happy life.
So you see, the thing that most limits what you can do is what you think in your own mind.