Tenets

The tenets are the rules the assassins live by:

1. Still your blade from the flesh of the innocents.

1. Always remain subtle.

1. Never endanger the brotherhood (in other words don't bring the templars back to Masyaf or endanger other assassins.)

Altaïr breaks all three of the tenets in the beginning of the game and as a result, lost his rank. If you kill an innocent you will lose some 'health' as it partially desynchronizes Desmond. If you kill too many innocents the desynchronization becomes too great and the screen will say: desynchronized, reason: killing innocents; first tenet.

There is also an achievement you can unlock if you don't break any tenets throughout the game.