Marginal Utility of Income

The idea that an extra dollar matters more to someone on a low income than someone already rich.
For a struggling family, $100 might mean groceries. For a billionaire, it’s… genuinely nothing.
This is one reason progressive taxation improves overall wellbeing in society. Money goes where it can produce more wellbeing.
Visual idea: $100 filling a nearly-empty cup vs barely changing an overflowing cup.