At first glance, the key and the lock it fits may seem very different. Different in shape, different in function, different in design. The man who looks at them without knowledge of their true nature might think them opposites, for one is meant to open, and the other to keep closed. Yet, upon closer examination he might see that without one, the other becomes useless. The wise man then sees that both lock and key were created for the same purpose.

— The Well of Ascension by Brandon Sanderson (Mistborn, #2)