"Nothing is true; everything is permitted." What does this really mean?

I love it when excerpts from things like video games inspires a question! This quote is from Assassin's Creed the game and is, in fact, the Creed:

Nothing is true, everything is permitted

First let's keep in mind that the world the game is set in has three basic factions:

• the Assassins, who are a small force composed of individuals that look to allow people the ability to have freedom

• the Templars, who are members within an hierarchical organization that looks to bring order and efficiency to the world by having people act like cogs in a wheel

• Everyone else

There are all sorts of sub-plots and things involving the finding of powerful artifacts and such, but for the purposes of this quote they are irrelevant.

Nothing is true, everything is permitted

The first thing to know is that this quote is not something that is a call to action but rather commentary on the way the world is. From the wiki article I found:

Ezio Auditore da Firenze once spoke of the maxim at length with Sofia Sartor, who found it rather cynical. However, he told her that the maxim was not a doctrine to be followed, but merely an observation of the world.

In detail, he explained that "To say that nothing is true, is to realize that the foundations of society are fragile, and that we must be the shepherds of our own civilization. To say that everything is permitted, is to understand that we are the architects of our actions, and that we must live with their consequences, whether glorious or tragic.

Also keep in mind that the Creed is not meant to stand on its own, but instead refers to Three Tenants and Three Ironies:

• Stay your blade from the flesh of an innocent
• Hide in plain sight
• Never compromise the Brotherhood

and

• The Assassins seek to promote peace, but commit murder
• The Assassins seek to open the minds of men, but require obedience to rules
• The Assassins seek to reveal the danger of blind faith, but practice it themselves

An Assassin would never, for example, use the Creed ("nothing is true") to say that there are no innocent lives and then use "everything is permitted" to do whatever s/he wanted to whomever they wanted. That type of cynicism towards people is the attitude of the Templars and is directly contrary to the attitude of the Assassins.

http://assassinscreed.wikia.com/wiki/The_Creed