The Assassin's Creed is based on the idea that nothing is absolutely certain and that individuals must be free to make their own choices.
The phrase "Nothing is true, everything is permitted" expresses this philosophy. It does not mean that reality does not exist or that people may behave without consequences. Instead, it reminds Assassins to question accepted beliefs, recognize that institutions can be manipulated, and understand that freedom requires responsibility.
Assassins are expected to think for themselves rather than blindly follow orders.