Full Idea
A cautious man who loves his own security will believe all men his friends, but liable at any time to become enemies.
Gist of Idea
Cautious men see everyone as a friend who could become an enemy
Source
Samuel Pufendorf (On the Duty of Man and Citizen [1673], II.01)
A Reaction
I'm looking at my friends in a different light after reading this one. This sounds like a strategy in game theory.
Book Reference
Pufendorf,Samuel: 'On the Duty of Man and Citizen' [CUP 1991], p.119