Post #11 in this thread called for the 49% of the population who currently don't pay income tax to begin coughing it up. That would include the poor, some of whom not only don't pay income tax but get an earned income tax credit.
I am against a flat tax because the wealthier one becomes the lower percentage of income must be spent on the necessities of life (food, shelter, health care, clothing, etc.) Because they have more disposable income (by definition income they can fritter away in any manner they choose,) I feel the fair thing is to expect them to pay a heavier burden in tax.
My belief is that when "defining" fair as many in this thread have inquired about, it is inherently more fair for those who make more money to be expected to pay more and a higher percentage of their income (regardless of how it is "earned") to taxes.
Ultimately this boils down to a philosophical difference between two camps and how they see "fair." One one side are those who believe that "fair" and "equal" are synonymous. On the other side is the camp that believes "fair" should be defined more closely with "best." Some people are really good at making money. Others are really good at sports or scholarly pursuits or engaging with other people. In life, it's naive to think that the playing field is equal for all. And it's not fair to hold those who aren't good at making money easily to the same standard as a person who finds it easy and even effortless to make a bunch of money, regardless of the circumstances.
It is folly to think that someone in one camp can convince someone in the other camp that their position is correct, regardless of how well though out and logical their arguments are. Just like it would be pointless for me to read a book written in a language other than English. I don't think that way and so I'm never going to be able to understand the other perspective.