what principle propogates that an individual’s tax bills should be directly propotional to his income?
You don’t have us doing your homework, do you?
Are you asking as a matter of philosophy or a matter of law?
As a matter of law in the United States it’s the 16th Amendment to the Constitution.
What do you understand the current income tax structure to be?
What would you propose to replace it?
Are you familiar with the current proposal for a ‘national’ sales tax?
What is your opinion of it?
Answer to OP is: The more you earn the more you pay.