So last year in my English class we read Hamlet, and the culminating essay involved the question whether or not the ghost of Old Hamlet is real. I stipulated a middle ground where the first two meetings with the ghost were real since they were during the witching hour. In the scene where Hamlet sees him in the middle of the day, I said the ghost was a hallucination or something similar. My question is: what was the cultural understanding (and Shakespeare's) of the time regarding hallucinations?
It’s a great question, but unfortunately there’s no concrete answer. Just like today, different beliefs about ghosts and the imagination could be found among different people. The biggest factor was probably religion; Hamlet offers a conundrum in that Hamlet and his friend Horatio follow a very Protestant line of thinking (and the fact that they go to school at Wittenberg–the famed site of Martin Luther’s 95 Theses–probably isn’t an accident), but the ghost himself basically explains that he’s in Purgatory, which is a distinctly Catholic concept. This conundrum has been stumping scholars for years, and it wasn’t simply artistic sloppiness, because Shakespeare has Horatio comment on it: in the early scenes, he warns Hamlet not to follow the ghost because it might be a trick conjured by the devil, which is a textbook Protestant interpretation. And Hamlet spends the next two acts of the play trying to determine whether or not the ghost is telling the truth. In the Mousetrap scene, he decides he is. Now, whether or not that means the ghost is really his father or a conjuration of the devil is still up for debate; consider Macbeth, wherein Banquo warns “…oftentimes, to win us to our harm, / The instruments of darkness tell us truths.” Again, a very Protestant idea. However, there’s also a reference in Macbeth (made again by Banquo) to hallucinogens: “Were such things here as we do speak about? / Or have we eaten on the insane root / That takes the reason prisoner?” And of course, in Midsummer there are quite a few mind-altering plants in play. Then of course there is the possibility that Hamlet is just seeing things: early in the play Horatio and Bernardo and Marcellus can also see the ghost; later in the closet scene Hamlet is the only one who can see it. But again, that also happens in Macbeth–Macbeth is the only one who can see Banquo’s ghost. Does that mean it’s a drug trip or a hallucination? We’ll probably never know.
So what was the cultural understanding? Depends on the culture. Depends on the circumstances. There are a few patterns with ghosts in early modern drama–they are almost always the ghosts of people who have died unjustly, and they almost always come back as a spectral manifestation of another character’s guilt–but there are no hard and fast rules.