The article explores how to decide the best way to tax people fairly while considering the impact of external factors on society. When there are multiple things causing these outside effects, like pollution or health issues, the taxes we need to put in place should also consider how these factors interact with each other. This means that when we decide on taxes for specific products or activities, we have to think about how they affect each other. The study shows that the relationships between these external factors are essential for making good tax policies, especially when there are more than one factor involved.