To nitpick: Why is Knowledge subjective? Knowledge just means familiarity with something. Now, whether it can be productive for you or not is subjective.
For Example: Say, you visit an ancient Mayan temple, learn about their customs and how they performed their rituals. You may or may not be able to take something out of it and enhance your personal life but certainly knowing in detail about it gave you a better position to make that judgement.
