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Interestingly, the first people to introduce a taxation system were the Athenians, about 2,600 years ago.
They had various tax rates, and these were all based on an individual's wealth, not on their earnings.
The lowest tax rate was zero.
Above that, the more land a person possessed, the more tax they paid, regardless whether it was cultivated or not, or how it was cultivated. Tax wasn't calculated in percentage of earnings, but in drachmas per measure of land.
If the Athenians didn't introduce a flat tax, then this can only mean that flat tax is nothing to take into consideration. What they essentially had was a wealth-based tax. These are the same people who conceived democracy.