I'll take a stab at this and suggest that he meant the law of diminishing marginal utility. Let's say I'm giving you $10 more to spend on your lunch today. If you've only got $1 in your pocket, that extra $10 can make a huge difference in your lunch. It makes less of difference if you've already got $20 in your pocket. And it makes next to no difference if you've already got $1,000 there.
The same applies to a flat tax on income. If you're making $20k a year, that $2k buys a lot in terms of basic necessities and can make the difference in whether you go hungry or have your electricity turned off. If you're making $2 million, that $200k isn't going to affect your lifestyle all that much at all.
The only way I could see a flat tax working is if it were actually much higher and with a poverty level based indexed exemption. For example, you could not tax any income below two and a half times the poverty level and then tax a flat 33% of any income above that. Based on 2011 HHS Poverty Guidelines, a single person making $50k a year would end up paying ~$7600 in taxes (around 15.2% which is less than the current marginal tax rate of 17.25%). Single people making $100k would be hit with a slight tax hike (24% vs 21.6%) as would people making $500k (31.5% vs 30.5%).
The biggest difference would be that without any other exemptions or deductions, the code would be greatly simplified.
That's what I figured.
OK, so money is theoretically worth "less" to someone who has more than someone who has less. I get that, I've been there. I only made around $20k last year, but $5 now is worth "less" to me than it was a few years ago when I was a poor college kid that maybe made $2000 for an entire year.
However, what I do NOT agree with is saying "well money isn't worth as much to people above x income level, so increase the tax rate on them" - That's bullcrap. Increasing the tax rate because it theoretically "doesn't matter as much to them" isn't right.
The only real fair tax is a flat tax rate. 10% of $1M is $100k - that guy would be paying more in taxes than I would dream of making in a year. And you say he isn't paying enough? What? How do you come to that conclusion? "Well he can afford to pay more" - So what if he theoretically can? Does that make it right? NO.
The main reason I think for tax brackets and crap is because the government can't be bothered to keep a decent budget, so to get more and more money to waste they tax people more and more. Of course, they don't dare tax everyone more, as that would be hard on the poorer people and make them very unpopular. But it is popular to tax the "evil rich people" even more, and hey now they have more money to waste! But even so, the government still can't be bothered to balance the budget even after grabbing billions of dollars from its citizens
Here's another thing - for the "evil rich people" who do use loopholes and stuff to basically avoid taxes on large amounts of income, well, close a bunch of the loopholes. Simplify the tax code. Easier math, less screwing around with getting the details right, and the tax revenue remains high enough. It doesn't have to be near as complicated and full of gotcha's and loopholes as it is now.
I do want to make it very clear that I am NOT in favor of an across the board 10% tax as suggested by the OP. That's retarded.