The agenda setting theory is based on the idea that the media influences the salient issues and images in the minds of the public through their choice in newsworthy stories.  According to Walter Lippmann, an American journalist and one of the first people to formulate this idea, the public responds to the images presented through media rather than to actual events in the environment: "For the real environment is altogether too big, too complex, and too fleeting for direct acquaintance.  We are not equipped to deal with [it]...And altogether we have to act in that environment, we have to reconstruct it on a simpler model before we can manage with it" (Littlejohn, 2002, 319).  Donald Shaw and Maxwell McCombs furthered this idea and believed that agenda setting occurs because the press has to be selective in their reporting of the news; thus those in charge make decisions about what to report and how to report it, ulitmately influencing the public's knowledge and perception of events.  This function is a three-part linear process comprised of a media agenda, public agenda, and policy agenda.  Within the media agenda, the priority of isses and topics to be discussed in the media are set.  The public agenda is then considered as the media agenda in some ways affects and interacts with the ideas of the public (public agenda).  Finally, in the policy agenda, the public affects and interacts with policymakers on what they consider to be the most important issues.  The process is thus interdependent and creates a system, though credibility also plays a role in determining the effect media has on the public (Littlejohn, 2002).