[stale2002]

It could still be an externality.

For example, let's say you put all your garbage out in the street in front of your house, blocking the road. This hurts BOTH you, because you can't get to work, AND other people who have to drive around the blocked street.

Even though the trash in the street hurts you the most, it is still an externality, because it is ALSO, hurting other people.