There are more things in heaven and earth, Horatio
Swear by my sword
Never to speak of this that you have heard.
[Beneath] Swear by his sword.
Well said, old mole, canst work i' th' earth so fast?
A worthy pioner! Once more remove, good friends.
O day and night, but this is wondrous strange!
And therefore as a stranger give it welcome.
There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.
Horatio and Marcellus, though advised against it, barge into
Hamlet's conversation with his father's ghost [see SOMETHING
IS ROTTEN IN THE STATE OF DENMARK]. Hamlet is a little
unforthcoming with the news imparted by this spirit, who is still
rustling about under the stage. So it's hard to figure what Horatio
and Marcellus are being asked to keep quiet, though Hamlet and the
burrowing ghost (a "pioner," or miner) insist.
Horatio, a model of rationality, is still having a hard time
swallowing the whole business. Ghosts are not the sort of beings
his "philosophy" easily takes into account. We know that Horatio
is, like Hamlet, a student at the University of Wittenberg, a
notable outpost of Protestant humanism. The philosophy he studies
there is probably classical—a compound of ethics, logic, and
natural science. The emphasis on everyday phenomena pretty much
excludes speculation about talking ghosts.
Wittenberg, however, isn't just a place where sober-minded
Horatios debate Aristotelian physics. In Christopher Marlowe's play
of the late 1580s, Doctor Faustus, it is where the doctor
lectures and, on the sideline, fraternizes with demons.