Economic theory. A rich person earns more money per hour than a poor person (by definition). Money is a representation of value (by definition). In Economics, value earned is at least proportional (if not equal) to value created. So a rich person creates more value in the same amount of time than a poor person. Their time is literally more valuable.
In theory there's no difference between theory and practice. In practice, there is.
In theory there's no difference between theory and practice. In practice, there is.