The idea that things are determined by fate. Instead of trying to change these things, you should accept them.
Your personality can and should exist the way you want it to, and not be dependant on other people or on the situation you are in.
In other words, if you are a slave, it is only because you see yourself be one.
If you are in prison, you are only confined if you believe you are -- to put it another way, you are the one who should define 'confined', and not leave the distinction up to your jailer. You would have had limits to what you could do under any situation; you could not go to the moon, try every dish in the world, or live forever. But if you think you can be happy with one set of highly restrictive limits (the life of a king, for example), why not with you current situation? Your perception is all that stands in the way of your happiness.
This was started by Zeno, but not that Zeno. The other one. Zeno of Cittium.