Well, you do need to think that you can do something before you can do it. That is the beginning of the possibility. But there are also a great number of factors that must be arrayed your way for the thing to happen, too. Some of those, you can solve. Some, you can’t.
For example, Jimmy in the ghetto and Howard Graham Buffet (Warren’s son) can both go on to work hard and become millionaires, I suppose. However, Jimmy’s going to face obstacles that Howard might not, such as finding it harder to get a job, an education, a different set of cultural expectations, and the resources needed to get a good start on his dream, whatever it is. Howard would have been born with a leg up. I suppose this is what people mean when they say that “we all have equal opportunity, but we are not guaranteed equal results” (although what they say they mean is that if Jimmy and Howard don’t both reach their millionaire dream, Jimmy just didn’t work hard enough).
Sometimes, Jimmy’s going to work hard, concentrate, and focus, but he’s just not going to have the resources or opportunities – so the American dream is theoretically possible for everyone, but in reality, many people are not going to make it.