The Left 4 Dead series uses a collection of artificial intelligence routines, collectively the "AI Director", to monitor and alter the gameplay experience in response to the players. Valve's primary goal with the AI Director was to promote replayability of the games' campaigns, as their previous multiplayer games with this feature, such as Counter-Strike and Team Fortress 2 have shown thriving communities of players that continue to play the games despite the limited number of maps available due to the unpredictable nature of online play.[21] In considering this for Left 4 Dead, Valve identified that many games use static events that always occur at fixed points in a level, or limited dynamic events where one of several events could occur at fixed points. Valve themselves had used this idea in some key battles in Half-Life 2: Episode Two where the spawning of Combine forces would be based on the player's location.[22] They recognized that such systems do not promote replayability or cooperation: players could easily memorize where events would occur, and those that had yet to experience the events would slow other players down.[21] With the concept of the AI Director, Valve believed it could capture the same chaos and randomness that would occur in Counter-Strike and Team Fortress 2 in the cooperative gameplay experience, transforming it from simple memorization to a skills challenge.[21] Valve has further termed this approach "procedural narrative", creating a new story each time the game is played.[23]