HAMLET
Why, then, ’tis none to you; for there is nothing
either good or bad, but thinking makes it so: to me
it is a prison.

ROSENCRANTZ
Why then, your ambition makes it one; ’tis too
narrow for your mind.

HAMLET
O God, I could be bounded in a nut shell and count
myself a king of infinite space, were it not that I
have bad dreams.

The line, ‘there is nothing either good or bad, but thinking makes it so,’ came up in conversation recently.

I remembered the bit about the nut shell, but the last line of exchange escaped until, while later, I was driving, and it hit. And when it did, I realized how much it changes to whole color of the dialogue (though putting squarely with the rest of Hamlet which is, surely, at least partly about the existence/nonexistence of free will, maybe not in a metaphysical sense, but in the sense of being trapped by external events).

Taken of itself, that line, ‘there is nothing either good or bad, but thinking makes it so,’ is a wonderful example to (horrible) parents (usually fathers) who want their children to stop being upset about something that has, quite naturally, upset them. Because, really, it’s their fault they are upset, because it’s only their thinking that has made it ‘bad.’

But ‘were it not that I have bad dreams’ changes everything, doesn’t it? Even though dreams are an internal thing, they are also (at least partly) external reflections. You can’t make the world good or bad just by thinking, because the world will always intrude. Yes, it may all ‘be in your head,’ but the world is also in your head and the world has its own ideas.