I know what you're thinking. You're thinking: Phil, you do go on about Stoicism, but where's the practical application? What's the takeaway? Well, let's take a look at one way Stoicism gives a practical and un-intuitive ethical result. Most people think that justice (roughly defined as 'just deserts', or making it so that individuals don't profit from immoral actions) is ethically desirable. Most people think that revenge (roughly defined as emotionally-motivated-justice) is ethically desirable in extreme circumstances. Stoicism, the way I construct it, classes both justice and revenge in the same category. On this view, justice and revenge aren't just obviously unethical, but motivated by a fundamental misunderstanding of the nature of harm.
Here's a brief review of the Stoic concept of harm. The only things that should be important to us, it says, are things we can control. We can only be harmed by damage to things important to us - 'no property, no harm', for all you anarchists in the audience. Since the only thing we can control is our own will, our own moral choices, then we can only be harmed by making immoral choices. Therefore, the only person who can harm us is ourselves.
What ramifications does this have for justice and revenge? Well, both exist as responses to harm, as ways of punishing the person who caused us unfair harm. But if we accept that nobody can cause us harm but ourselves, then we can only justifiably take revenge or exact justice on ourselves! Back up another moment: both justice and revenge seek to even the scales, to ensure nobody profits from sin. If we harm ourselves, then that harm is punishment enough for having caused it, and no further retribution is needed. So, on the Stoic view, there's no instance of permissible revenge or justice.
I'll set out a little example to show how this might work in real life. Stoical Sam is summoned before a mighty king, who commands Sam to go forth and assassinate a rival's children. Sam, being a stand-up guy, refuses, and is tortured. Now, for Sam, the torture itself doesn't consitute harm - it's a necessary consequence of the moral decision not to kill children, that's all - and the only potential harm in the situation is the possibility of him giving in and acting in an immoral way. In fact, the torture gives him a chance to demonstrate his moral fortitude, and practice the important skill of bearing up under pressure. The king's rival executes a stunning coup and frees the prisoners, including Sam, who is given the chance to decide what will happen to the king.
What happens if Sam walks away, deciding not to punish the king at all? Nothing - and it's a reasonable response, since from Sam's perspective the king has (unwittingly) done him a service.
What happens if Sam decides to have the king tortured? Sam becomes a torturer, thereby harming himself (his soul, if you will) irreparably. Worse still, from a Stoic perspective, he doesn't harm the king at all through doing this.
And what about practical applications today? Turns out I was right to tell you not to call the cops! So there.