Iron sharpens iron; scholar, the scholar.
As iron sharpens iron, so a friend sharpens a friend.
One must dig deeply into opposing points of view in order to know whether your own position remains defensible. Iron sharpens iron.
It's iron that can sharpen iron; wood cannot do that.
Iron rusts from disuse; water loses its purity from stagnation... even so does inaction sap the vigor of the mind.
Leonardo da Vinci
Want to accomplish something, any-damn-thing? Sharpen your political skills!