Libertarians only recognize one valid constraint on your actions: A universal, mutual constraint against aggression, also known as initiation of force.
The simple version, courtesy of Matt Kibbe: Don't hurt people, and don't take their stuff.
When you throw the first punch, or pick someone's pocket, or otherwise forcibly interpose yourself between someone else and that someone's life, liberty, or property, you're not running your own life. You're trying to run theirs.
And that's the only thing libertarians agree you should be stopped from doing or penalized (in a manner consistent with restitution, not "punishment") for doing. Even if it's "for their own good."
If you're down with that idea, congratulations: You're a libertarian.
If you're not down with that idea, I hope you'll think it through more carefully.
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