Initially, Scrooge believes that the appearance of Marley's ghost is a result of indigestion. However, once the ghost has gotten Scrooge to take the first steps toward understanding that the ghost is indeed "real," Scrooge begins to think of other reasons as to why it has appeared.
As he continues to talk with Marley's ghost, it says, “I am here to-night to warn you, that you have yet a chance and hope of escaping my fate. A chance and hope of my procuring, Ebenezer.” To which Scrooge replies "You were always a good friend to me . . . Thank’ee!”
Here Scrooge discerns another reason for the presence: his old friendship with Marley. In this moment, Marley's desire to help out his old friend has caused the ghost to appear. However, when the ghost tells him about the three spirits yet to come, "Scrooge’s countenance fell almost as low as the Ghost’s had done." Scrooge's understanding begins to shift again. This is no kind act of friendship; this is something more. However, Scrooge's new understanding holds for only a brief moment, as in stave 2 Scrooge's perspective again shifts as his mind again attempts to rationalize the ghost away.
Scrooge went to bed again, and thought, and thought, and thought it over and over and over, and could make nothing of it. The more he thought, the more perplexed he was; and the more he endeavoured not to think, the more he thought.
Marley’s Ghost bothered him exceedingly. Every time he resolved within himself, after mature inquiry, that it was all a dream, his mind flew back again, like a strong spring released, to its first position, and presented the same problem to be worked all through, “Was it a dream or not?”
Here Scrooge again has trouble believing that the ghost has even appeared at all, in some ways returning to his initial belief that the vision was all the result of indigestion. However, this perspective is soon obliterated once and for all as Scrooge meets the first of the three spirits and revisits his own past.