User blog comment:Smijes08/Arkham Knight: Joker- Ghost or Hallucination?/@comment-26833018-20151011115023

Although the Joker hallucinations were bought about by Scarecrow's Fear Toxic and represent Batman's fears, they also represent the Joker Infection's influence on Batman. The more Joker appears before Batman, the stronger the influence is until these thoughts develop into a alternate personally that eventually takes control of Batman's body completely.