There is nothing either good or bad, but thinking makes it so.
Hamlet says this to his old schoolfellows Rosencrantz and Guildenstern while they argue about whether Denmark is a prison. It is spoken by a character with his own reasons for saying it, and it echoes an idea the Stoics, such as Epictetus, had put forward long before.
Cowards die many times before their deaths; / The valiant never taste of death but once.
For thy sweet love remembered such wealth brings / That then I scorn to change my state with kings.
I love you with so much of my heart that none is left to protest.
If you live according to nature, you will never be poor; if you live according to opinion, you will never be rich.
It is not the man who has too little, but the man who craves more, that is poor.
If thou art pained by any external thing, it is not this thing that disturbs thee, but thy own judgement about it. And it is in thy power to wipe out this judgement now.