The Ghost represents an actual or symbolic transcendental reality.  He proclaims, through his charge to Hamlet, and in his own purgatory, that there is justice and morality.   

Of course, the Ghost himself is not an objective personality.   He is being punished in purgatory fo his own sins.   He has a personal interest in avenging his brother's crimes against him.  He may not even be who he claims to be.  (Click here)

Although it is debatable whether the Ghost exists only in Hamlet's mind in III.iv.104 et seq. there is little question that the Ghost in I.i and I.iv is physically present within the play.

More neutrality:

In Ham.

In the Vale

It is not simply a question of whether Hamlet is mad, but whethet the author is mad and whether we the audience are also mad.  Have we left reason behind to believe that TRUTH, exists within, or without, the motal bounds, that there is Justice, Morality, beyond Nietchze's "Forging of Belief"?  

The play, answers neither rhetorically nor philosophically, suggesting that these are "thoughts beyond the reaches of our souls . . . "  I.iv.54-56.   Oscilation between the rhetoric and philosophies in the play reveals the nature of the question: it is unsolvable and always with us.   "The rest is silence. "  As such, the play reveals the bias of the author -- his belief that there is ultimately no human answer. Or does it?