This quote means that the essence of true discovery is an internal one. A person does not need to be an explorer or conquer the world over in order to be worth of "discovery." It simply involves being able to fundamentally see and think things that others have not been able to formulate. Think of a chess game between two people. They stare at the board for a while and contemplate their next move in order to be victorious. They stare and continue to stare until someone from the outside takes a gander at the game and "sees" a sequence of moves that one player can take to easily checkmate the other. The outsider represents this concept of "discovery" because they "see" what others "see" and "think" what no one, in this case the two players in our chess game," had been able to think before. This is where discovery rests. The quote suggests that discovery is in the heart and mind of each person, in the capacity of each individual in order to transform their world.