We can learn about this principle (surprisingly) from a jar full of fleas. If you fill a jar with fleas and place the lid on the jar, the fleas will jump and hit their heads on the lid. But leave them for a few days, and the fleas will adapt to their environment and, to avoid the pain of hitting their heads, eventually will not to jump as high as the lid
So, what do we learn from these fleas? Just as the fleas began with the capacity to jump higher than the lid of the jar, we as human beings have the capacity to accomplish great things. However, through our experiences and our own reasoning, we sometimes come to believe that our ability to succeed is limited.
This applies in all areas of life. I have several friends that I know have the capacity to achieve great success. But I find them using phrases like "I guess I missed the boat on getting an education," or "I could never accomplish as much as (that person) has accomplished." These beliefs about themselves and their capacity to succeed limit them in their learning and in their lives. The beliefs may have begun based on some sort of experience, (like me struggling with maths classes at school), but a temporary setback should never be interpreted as a life-long ultimatum.
"Do just once what others say you can't do, and you will never pay attention to their limitations again." - James Cook