Most of us never heard the phrase “untaxed externality” until Elon Musk used it during a talk he gave at the Sorbonne last December in Paris. It is an arcane term that economists use. Essentially, it refers to a cost created by one person that has to be paid by another. Carbon dioxide is one such untaxed externality. It is a product of virtually all human activity that involves manufacturing or transportation. It is inherent in the use of fossil fuels and it is a primary cause of climate change.