“There is neither good or bad but thinking makes it so.”
This is a line from Hamlet. It comes during an exchange in which Hamlet states that he sees Denmark as a prison, due to his circumstance, but his friend sees it differently.
For me this is the essence of Stoicism. When two people can interpret the same reality in completely different ways we know that the emotional reaction must be happening within our minds. This realisation comes along with a great deal of power; the reality of the world is not absolute but is determined in your mind, if you can control that reaction you can literally change the world.