> Another example of externalities would be a widget factory that spews carcinogenic chemicals into the air. When I trade with the widget factory I'm benefitting - I get widgets - and they're benefitting - they get money. But the people who breathe in the carcinogenic chemicals weren't consulted in the trade.<p>Well, actually, polluting the air is not a problem by itself, of course... but there is another perspective: if you're spewing chemicals into the air, you are forcedly taking the rights in lungs, fields, and other property of people living nearby.<p>That's an act of aggression, and therefore libertarianism is against it. But the article makes it sound as if "voluntary exchange" is all libertarianism is about, ignoring nonaggression principle.