While we are meant to believe that the visitation of the ghosts is actually happening, it is perhaps more important to think of themand the scenes they reveal of Scrooge's lifeas products of Scrooge's imagination. Provoked by the sudden thought in his old age that his life has possibly been for naught, he reconsiders what Christmas means to him.
This type of instantaneous, life-changing thought can be called an epiphany, and Dickens suggests that epiphanies require the mind to integrate all three major tenses; the past, present, and future, into a coherent, unified tense. For all intents and purposes, it does not matter that the Ghost of Christmas Past has visited Scrooge; Scrooge may simply be reliving his life through his memory, and the Ghost is merely a convenient symbol for memory. (Indeed, the Ghost looks like both an old man and a child, underscoring the elderly Scrooge's flashback to his childhood.)