Nothing either good or bad, but thinking makes it so
What have you, my good friends, deserv'd at the hands of
Fortune, that she sends you to prison hither?
Prison, my lord?
Denmark's a prison.
Then is the world one.
A goodly one, in which there are many confines, wards, and
dungeons, Denmark being one o' th' worst.
We think not so, my lord.
Why then 'tis none to you; for there is nothing either good
bad, but thinking makes it so. To me it is a prison.
What brings Rosencrantz and Guildenstern—two of Hamlet's
acquaintances from the university—to Denmark isn't Lady Fortune
but, as Hamlet suspects, King Claudius. Claudius is worried about
Hamlet's seeming distraction, thinking it might be a threat to the
state and to the king himself. Claudius coerces Rosencrantz and
Guildenstern, who aren't too bright, into service as spies, hoping
they can lull the prince into revealing the true cause of his
"antic disposition" [see ANTIC DISPOSITION].
When Hamlet calls Denmark a prison, therefore, the metaphor is
apt. He is mentally and physically confined by the gaze of the king
and his agents, and he feels trapped in the court's general
degradation—"Something is rotten in the state of Denmark," as
Marcellus had said [see SOMETHING IS ROTTEN IN THE STATE OF DENMARK].
Hamlet is a prisoner of his own thinking, and of his knowledge
that his stepfather is a fratricide and his mother incestuous. When
he states that "there is nothing either good or bad, but thinking
makes it so," he's not indulging in ethical relativism as much as
wishing for blissful ignorance. He's also implicitly damning the
naïveté of the king's new yes-men.