Taxes - A Fixed Amount

Imagine two twins with an identical life. They both had a happy childhood and the same education. When adulthood comes they get the same job but one of them, who likes to own more stuff, goes to work 8 hours a day and the other, that likes more free time, goes to work 6 hours. After a week of work they both are called, as everyone else, to help build a road for the community. The brother that has worked 8 hours a day is asked to work 2 hours and the other one is asked to work just 1.5 hours. Does this sounds fair? Taxes shouldn't be progressive, they shouldn't be as a percentage of income. Ultimately they should be a fixed amount per person.
Imagine going to the cinema and be asked how much you earn so they can know how much to charge you for the ticket. Cinema tickets are a fixed amount, they do not depend on your income.

Uncertainty

Everything else constant, yesterday is preferred to tomorrow. Yesterday is certain, tomorrow isn't.