Who Doubted there had even been a ghost appearing to the guards?
In Act 1 scene 1 of the play, the guards on platform of Elsinore castle are on their watch when Marcellus and Horatio arrive on the scene. The guards, Francisco and Bernardo think they have seen a ghost the past two nights, but they are bringing Horatio up the platform in the hopes that he can confirm the sighting. The reason they do this is because Horatio is a scholar and a friend of Hamlet's who is with him home from Wittenberg. Horatio, as a scholar, is very skeptical of the whole story and while up there even says, "tush, tush, 'twill not appear." But once he sees the ghost with his own eyes and tries to command it to speak, he is a true believer in the veracity of the ghost. He explains, "Before my God, I might not this believe / Without the sensible and true avouch / Of mine own eyes." He was a doubter, but now that he has seen it with his own eyes, he can't deny the truth, and in fact he talks specifically about what the ghost looks like and makes the connection to the ghost looking like the late King Hamlet who was once dressed for battle in clothes like the ghost is wearing. He concludes this encounter with the statement that foreshadows the conflicts of the play: "This (appearance of the ghost) bodes some strange eruption to our state." The appearance of a ghost is never something to be welcomed.