The Lever

While hiking in the woods, Nate and Sam found this huge rock which had an old iron lever attached to it. Etched into the rock was the following inscription:

"If this lever is pulled, the world will come to an end!"

Nate wanted to pull the lever and see what would happen, but Sam, being a paranoid pessamist, greatly feared this! He said to Nate that if he tried to pull the lever, he'd shoot him! In a daring attempt, Nate lunged for the lever, and sure enough, Sam shot him!

What is the moral of this story?