“The research problem is like a gold mine. Once the mine has been there for a long time, you need to mine/dig deeper and deeper to find a piece of gold. If a research problem has been exhausted, you have to work harder to find something new.”
The dilemma: It is difficult to find new solutions to classic research problems since many researchers had already studied the problem. Nevertheless, new research problems are also difficult because you have to establish the path to the mine before finding the gold, no one to follow or seek help from.
The moral of the story: Intelligence can get you a PhD but luck can lead you to the gold faster than your or your adviser’s expectation.
Recommended reading: Outliers: The Story of Success by Malcolm Gladwell