So far, so good. When it comes to the big ways in which we aggress against each other, libertarianism’s absolute prohibition seems plausible on its face.

But what about the smaller ways? Suppose I aggress against you not by beating you over the head with a club, but by blowing tobacco smoke into your face? The smoke‐​blowing, just like the clubbing, is a physical invasion of your body. And it is a harmful invasion. Not, perhaps, as immediate or as severe a harm as the clubbing, but the difference is only one of degree, not of kind. Stealing a nickel from my piggy bank is just as much an act of theft, and therefore just as much a violation of my rights, as stealing $100. If, then, aggression is to be prohibited absolutely, it seems that consistency will force us to prohibit not just big aggressions like beatings and grand theft, but little ones like smoke‐​blowing too.

Few libertarians have adequately come to terms with the full implications of this point. One of the earliest attempts to do so can be found in Murray Rothbard’s popular work, For a New Liberty . Chapter 13 of that book contains an extended discussion of the problem of environmental issues, and in it Rothbard makes the point that industrial pollution is a form of aggression, and thus impermissible according to basic libertarian principle.