I understand the argument for all involuntary tax being unjust... Once you accept the idea that government can coerce people into paying tax at all, you have already made a jump toward realism rather than principles... Ideally taxes would be voluntary... just wouldn't work because of free-riders.
Here's a problem with an actual flat tax amount:
The government is going to end up either not having enough revenue to even supply a decent defense... or it is going to use a lot of revenue bailing out the people it has bankrupted. And what happens with people who make less than the tax rate?
It would end up having the effect of reverse income redistribution... The rich would easily absorb the impact... the middle class would be stagnant... the pooor would be looting the middle class because they're in debt to the government... or we have a totally ineffective government. One or the other.
The other principle behind a tax rate, also used for progressive taxing, is that the wealthier you are the more benefit you have from being protected by the state (tend to have better schools and more stuff to defend and all that stuff)... but like I said, once we accept taxes we have moved toward practicality rather than principle.
If you want to be principled, voluntary taxes are it. The closest thing to a voluntary tax system that might work is the fairtax, because you have control over how much tax you pay by monitoring consumption.

And to think at one point our ancestors would have been happy just to have representation for their taxation. Sheesh.

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"Man lives in the sunlit world of that which he believes to be reality. But unseen by most is an underworld, a place that is just as real... but not as brightly lit... A DARK SIDE!"
-opening from Tales From the Darkside

Just think it over. A flat tax of - just as an example - 500 $ a month would generate a total tax amount for the US of about 1.5 trillion $ a year. 500 $ per month is affordable by everybody - and do not forget that there would be no other tax (VAT, State taxes, gasoline tax and so on) at all.

So where do you see the problem?

$500 per month comes to $6000. Rent comes on average, at least around here, to about $8400. Add that, we're talking $14,400. OK. Now when are taxes necessary? At 18 or from birth. Because when you add a wife, definitely adding $6000... a kid, either nothing or $6000.
So between tax and rent, you're looking at between $20,400 and $26,400.
The median household income last I new was around 30-40,000... and a lot of people make less than that with families of three or more.
For instance I can think of at least one couple with a kid where the man is a breadwinner and makes around $20,000.
So for that family, we're saying... before food... they are in debt to the government... or living on the street.
A lot of studies show that the costs of another parent working (especially one with no specific skill or education) would outweigh the income from that job... so the wife getting a job would probably only lead to a harder time raising a decent kid.
Meanwhile a multibillionaire pays $6000 and doesn't even notice.

Somehow this does not strike me as a fair or efficient system.

__________________
"Man lives in the sunlit world of that which he believes to be reality. But unseen by most is an underworld, a place that is just as real... but not as brightly lit... A DARK SIDE!"
-opening from Tales From the Darkside

Life isn't »fair« at all - and justice is neither. A truly just taxation would necessarily not be »fair«.

The system we have today is nothing more but a subsidy for social classes whose existence is not really necessary for the common welfare.

As it is today people who don't pay tax at all have a vote. With this vote they can elect politicians with a »social conscience« that rob the diligent to make unearned presents to the lazybones and/or stupid.

So at least people should have political rights adequate to the amount of tax they pay. Those who pay much should have much votes, those who pay nothing should not have a vote at all.

That would be just. The taxation systems in the western world of today are just insane.

So people who can get by now perfectly fine and responsibly on little money, which they work for, are now "useless classes" that should be taxed out of existence or lose their right to vote?
Doesn't sound just to me.
Frankly, being that I don't trust the wealthy any more than the poor, if such a system existed, I'd expect people in power to do whatever they could to keep their workers and anyone else with differing interests from being able to make enough to pay taxes.
I suppose checks and balances don't exist in your definition of justice, do they?
Sounds like fascism with a friendly face.

__________________
"Man lives in the sunlit world of that which he believes to be reality. But unseen by most is an underworld, a place that is just as real... but not as brightly lit... A DARK SIDE!"
-opening from Tales From the Darkside

So you think it is just that the government takes away your hard-earned money to give it to others that did not work so hard or did not work at all? To me this system appears als blatantly unjust socialism.

That would be an argument against welfare, not for a flat cost taxation.
Your tax scheme would starve out or disenfranchise a lot of working folks.
I won't even get into the public goods your overlooking (there are a few involved in "giving to those who don't work", as you put it)... It's irrelevant when the tax system itself would create an oligarchy.

__________________
"Man lives in the sunlit world of that which he believes to be reality. But unseen by most is an underworld, a place that is just as real... but not as brightly lit... A DARK SIDE!"
-opening from Tales From the Darkside

That is correct.

You are wrong. If everybody has the same rights, why the hell should some people have more duties and pay more tax? Same rights, same duties - everything else is not just.

This you cannot prove.

I already did. I added up the figures for a family of three... just taxes and rent. So a family with a household income of $20,000 would either be on the street or in debt to the government.

__________________
"Man lives in the sunlit world of that which he believes to be reality. But unseen by most is an underworld, a place that is just as real... but not as brightly lit... A DARK SIDE!"
-opening from Tales From the Darkside

No. The wife would just have to work, too - and so double the income. That would be sufficient for the tax burden - especially as the cost of living would be significantly lower than today as there would be no other tax (e.g. VAT or state tax) at all.

I do not understand why you think other taxpayers should subsidize the lazyness of certain people who do not want to work.